The original meaning of the phrase to hitch one’s wagon to a star is: to set oneself high aspirations.

However, in later usage, this phrase has come to also mean, cynically: to advance one’s ambitions by associating oneself with somebody more successful or powerful.

The phrase to hitch one’s wagon to a star was coined, in reference to idealistic aspirations, by the U.S. poet, essayist and philosopher Ralph Waldo Emerson (1803-1882) in American Civilization, an essay published in The Atlantic Monthly. A Magazine of Literature, Art, and Politics (Boston (Massachusetts): Ticknor and Fields) of April 1862 [Vol. 9, No. 54, page 505, columns 1 & 2]:

That is the wisdom of a man, in every instance of his labor, to hitch his wagon to a star, and see his chore done by the gods themselves. That is the way we are strong, by borrowing the might of the elements. The forces of steam, gravity, galvanism, light, magnets, wind, fire, serve us day by day, and cost us nothing.
[…]
And as our handiworks borrow the elements, so all our social and political action leans on principles. To accomplish anything excellent, the will must work for catholic and universal ends. A puny creature walled in on every side, as Donne wrote,—
——“unless above himself he can
Erect himself, how poor a thing is man!”
but when his will leans on a principle, when he is the vehicle of ideas, he borrows their omnipotence. […] Hitch your wagon to a star. Let us not fag in paltry works which serve our pot and bag alone. Let us not lie and steal. No god will help. We shall find all their teams going the other way,—Charles’s Wain, Great Bear, Orion, Leo, Hercules:—every god will leave us. Work rather for those interests which the divinities honor and promote,—justice, love, freedom, knowledge, utility.
If we can thus ride in Olympian chariots by putting our works in the path of the celestial circuits, we can harness also evil agents, the powers of darkness, and force them to serve against their will the ends of wisdom and virtue.

The earliest occurrences of the phrase to hitch one’s wagon to a star that I have found are as follows, in chronological order:

1-: From The Golden Hour (Boston (Massachusetts): Ticknor and Fields, 1862), by the U.S. clergyman, author and abolitionist Moncure Daniel Conway (1832-1907) [page 61]—Ralph Waldo Emerson was called the Sage of Concord:

X.
HOW TO HITCH YOUR WAGON TO A STAR.

It is one of the signs of the times, that the revolution was strong enough to take up bodily the Sage of Concord, and set him in the capital of this nation to instruct our rulers. The advice he gave them may be summed up in the one sentence, Hitch your wagon to a star!
Why not, Mr. President! You have some difficulty in making things go, possibly have some doubt as to whether they can be made to go; but if you could manage to hitch the Union to a star, that will be sure to move. If you can get the LAWS OF NATURE to aid in the reunion of North and South, you need not fear any Confederate efforts at keeping them apart.

2-: From An Unfortunate Farmer, published in the Ohio Cultivator (Columbus, Ohio, USA) of Monday 1^st December 1862 [Vol. 18, No. 12, page 363, column 2]:

Nature is the friend of every man. “Fortune favors the brave.” “The sun sihnes [sic] on the just and the unjust.” The lucky man is the man who understands the laws of lucks. Emerson has it when he says if you would succeed, “hitch your wagon to a star.” “The star” is going your way, and can draw it “just as well as not.” The simple English of this is, do your work according to the rules of nature.

3 & 4-: From accounts of lectures given from December 1862 to November 1863 by the U.S. humorist Charles Farrar Browne (1834-1867), whose pen-name was Artemus Ward—the following are two of many occurrences, apparently intended to be witty:

3-: From an account of the lecture given on Wednesday 3^rd December 1862 at Young Men’s Hall, Detroit, published in The Detroit Free Press (Detroit, Michigan, USA) of Thursday 4^th December 1862 [Vol. 26, No. 161, page 1, column 2]—Edward Everett (1794-1865) was a U.S. politician and orator:

The question was, what should he lecture upon? He could not write any such lecture as the Hon. Edward Everett, and he did not believe that Everett could write any such lecture as he could. He inclined favorably to astronomy until he read one of Ralph Waldo Emerson’s essays, in which he told the reader to hitch his wagon to a star. Candidly speaking, however, he did not feel himself competent to discuss the subject of astronomy in a lucid manner; if he did, he could have described a great many very ornamental constellations, among the most interesting of which, just at the present time, is the Southern Cross, which is supposed to be a mulatto.

4-: From an account of the lecture given on Monday 5^th January 1863 at Smith & Ditson’s Hall, Cincinnati, published in the Cincinnati Daily Enquirer (Cincinnati, Ohio, USA) of Tuesday 6^th January 1863 [Vol. 26, No. 55, page 3, column 7]:

The speaker introduced his lecture with a humorous allusion to the difficulties in the way of choosing a theme to discourse upon. He could not write any such lecture as the Hon. Edward Everett, and between himself and the hearer, he didn’t believe the Hon. Edward Everett could write any such lecture as he could. He could not lecture upon Astronomy after hearing Ralph Waldo Emerson speak of hitching “his wagon to a star.” “Now I prefer to hitch my wagon to a post, though, after all, there is no particular necessity of hitching your wagon to any thing, as a wagon very seldom runs off unless a horse is attached to it, and if a horse is sincerely attached to a wagon, he won’t run off with it.”

5-: From a letter to the Editor, dated Washington, Tuesday 21^st April 1863, by ‘T. S. P.’, published in the Portland Daily Press (Portland, Maine, USA) of Friday 24^th April 1863 [Vol. 1, No. 259, page 2, column 2]:

If we are at last prepared to follow without reserve or hesitation the policy of freeing and arming all the loyal black men of this country, if we are prepared to avail ourselves of all the advantages which the forever accursed system of Southern slavery puts within our power—if we are ready to enlist for our cause all the hopes and aspirations for freedom of four million allies, then we may reasonably look for better and brighter days. Thus do we avail ourselves of a principle as mighty, as it is pure and lofty—thus do we “hitch our wagon to a star.”

6-: From a review of Speeches, Lectures, and Letters (Boston (Massachusetts): James Redpath, 1863), by the U.S. civil rights activist Wendell Phillips (1811-1884)—review by the U.S. journalist and politician Horace Greeley (1811-1872), published in The Liberator (Boston, Massachusetts, USA) of Friday 4^th September 1863 [Vol. 33, No. 36, page 142, column 2]—reprinted from the New York Independent:

Mr. Phillips overestimates the importance of the part played by himself and his little band in the great drama now approaching its consummation. These sworn foes of Phariseeism have a most Pharisaic conceit of their own work and its consequences, which misleads and unduly inflates them. They assail or else condescendingly patronize men who have been as faithful to their light and as useful in their sphere as themselves. This conceit often distorts and exhibits them to disadvantage. “Hitch your wagon to a star,” is wholesome advice; but if you should happen to get the notion into your head that you are drawing the star, instead of being drawn by it, you will probably be led into mischievous phantasies and pernicious eccentricities.