This site is the #1 search result for funny lists and as a result I get a continual stream of new submissions. I’ve noticed a few common mistakes that could easily be avoided if people understood the correct usage. Here are a few pitfalls to be aware of.

They’re is the contraction of they are. “They’re funny.”
Their is the possessive form of they. “Their party was great.”

Too denotes excess. “She worries too much.”
To has lots of meanings but usually means towards or in order to. “She went to the store to buy some bread.”

Affect is usually a verb. “The vote didn’t affect the outcome.”
Effect is usually a noun. “The overall effect was breathtaking.”

It’s is the contraction of it is. “It’s a nice day today.”
Its is the possessive form of the pronoun it. “Life has its ups and downs.”

No one is perfect, so proofreading is a good way to catch your mistakes. A spellchecker is also helpful, especially if you’re a habitually poor speller.