You might want to invest in an Accuquilt Go! die cutter. This is a really good home system for people who have difficulty making accurate cuts with a rotary cutter.
A much cheaper thing to try out first (for straight strips) would be a June Tailor Shape Cut mat. This is a flexible mat with cut-outs to hold your rotary cutter in place as you roll. If your problem is with how you are holding the rotary cutter (the blade should not be tilted as you roll), it will show up when you use this mat.