Suspenders are traditional menswear accessories, now relegated to more formal use. This presents a potential problem if you're eyeing that new pair of slacks that don't feature any other apparent method of support. However, dress slacks with and without belt loops may indeed be properly kept in place using a variety of methods -- all without the need for braces.
Adjust the side tabs on your dress pants. These offer two to three separate button holes on each tab, letting you fine-tune your fit. These types of pants typically do not feature belt loops -- and look better with suspenders when and if you do decide to break them out.
Use a belt. Do not wear a belt if the pants do not feature loops, and try to always wear one if they do. Remember that if you're cinching the pants closed to the point where they visibly pucker, you need to have the trousers altered or move to a new pair.
Have your pants professionally altered to your waist. This is not very forgiving if you're prone to dramatic weight shifts, but allows pants without belt loops to be worn perfectly. Even pants with belt loops should be perfectly fitted, so the belt gives a little extra support without gathering excess material around your waist.
Wearing a belt and suspenders is generally considered a style no-no. Pick one.