Here are some more formal ways to say "suck up":
* Ingratiate oneself: This is a more neutral term, but it still suggests that someone is trying to gain favor through flattery.
* Currying favor: This phrase implies that someone is trying to win someone's approval by being excessively polite or helpful.
* Brown-nose: This is another slang term, similar to "suck up" but more vulgar.
* Toady: This is a noun that refers to someone who flatters someone in power.
* Bootlick: This is a very harsh and negative term that refers to someone who is excessively subservient.
The best way to describe "suck up" in proper English depends on the specific context and the level of formality you want to achieve.