What is Benvolio's advice to Romeo?

In general
terms,advisesto cure his obsession with the unattainable Rosaline by looking at other beautiful
ladies. But if you look at the text closely, you'll see he does a bit more than this.

First, he hints that any passionate love on Romeo's part is
probably undesirable. If he should fall in love with another lady, that would amount to an
afflictionjust like his love for Rosaline. The implication is that Romeo would be better off if
he didn't get so swept up in romantic feelings.

Second, he doesn't just tell
Romeo to go out looking. In Act I, Scene ii, Benvolio refines his idea to include a live,
side-by-side comparison of Rosaline with someone else. He suggests that Romeo only thinks
Rosaline is very beautiful because he saw her in isolation. When he sees her in the same room
with the competition, he will revise his views.

As noted by others who have
answered this question, Benvolio's initial advice occurs in Act I, Scene i. He tells Romeo to
stop thinking about Rosaline. Then, when Romeo asks how, Benvolio answers that Romeo should turn
his attention towards other beautiful ladies:

By giving
liberty unto thine eyes;
Examine other beauties

But if we skip ahead to the second scene of Act I we find more. There Benvolio
reiterates his theme, but now he presents it using metaphors about fire, pain, and
disease:

Tut, man, one fire burns out another's
burning,
One pain is lessen'd by another's anguish;
Turn giddy, and be holp
by backward turning;
One desperate grief cures with another's languish:
Take
thou some new infection to thy eye,
And the rank poison of the old will die.

Next, Benvolio seizes on the opportunity of the party. Rosaline
will be there, and so will "all the admired beauties of Verona." This is where
Benvolio tells Romeo to make those direct comparisons. He assures Romeo that Rosaline will seem
unattractive when they are done:

Go thither; and, with
unattainted eye,
Compare her face with some that I shall show,
And I will
make thee think thy swan a crow.

Finally, Benvolio
suggests that Romeo has overrated Rosaline's appearance because he viewed her in isolation
("Tut, you saw her fair, none else being by"). We may interpret this as additional
advice from Benvolio: Don't fixate on someone until you've actively compared her with some
alternatives.