- Write out any conjugation tables you're unsure of in Iversen's "green sheet" format: Just write out all the endings a few sheets of paper, and leave out the verb stems for the most part. Feel free to use different colors for surprising exceptions, or anything else that helps.
- Every week or so, just pick out a new table (say, the imperfect) and just memorize the endings.
- Use a spell checker, or if you're low tech, buy a copy of Bescherelle or BLED and leave it next to your desk. Even native French speakers look stuff up frequently! And honestly, a lot of them make mistakes if they're not writing formally.
I was actually working on a website at one point, which laid out the top 40 conjugation patterns or so, with notes about which verbs belonged to which pattern, and short lists of just the exceptional forms. The whole thing was machine-verified, too. I really ought to finish it. :-/