Just about everything people write is an opinion, so why not write in a manner that skips those formalities and gets straight to the point? It might come off as more absolutist, but should make for better writing.
Of course, there are times that you should use a qualifying "I think" or "in my opinion", but when they are absolutely necessary isn't clear to me.
"I think" is only for the purposes of softening a statement, which is sometimes (but not always) what you want.
Another tip that I got as a high-schooler was to not use "very" or "extremely". That also "dilutes your prose" (his words, which were apt). That came up in a recent NY Times Book Review. If you put "very" in front of every adjective, you sound like a high school student who hasn't been corrected yet :)
A generalized version of this is to never use an adjective/adverb/etc. unless it in some sense contradicts the thing it's applied to. There's no sense just trying to intensify the thing you've already written; the modifier should add new information.
I sometimes say explicitly when I'm on the fence about an idea ("Mild preference for A because __, though B would be ok"), or if I feel like my currently viewpoint isn't well-considered yet, to encourage others to read with a critical eye -- otherwise the polite default is often to go with the flow if there are no obvious or serious problems, and that can be bad.
