How can Illusion Magic be effective against True Sight?

I am starting a new campaign and decided to play a wizard this time around. I was looking through the different schools available and reading guides (Treantmonk’s among others) and saw illusion as one of the most liked ones. However, my DM and one of the other players have tried to dissuade me from this choice as a lot of higher level creatures (our last campaign went from 1 – 20) have true sight thereby making the illusions worthless.

My question is: How can an illusion wizard still be effective against creatures with true sight for a campaign that will extend to level 20?