How do you pull in your abs when exercising?

Practice breathing: A great way to understand how to pull in your abs while exercising is to do "abdominal" breathing. When you breathe in let your stomach muscles relax and your stomach will bulge outward. When you breathe out/exhale pull your stomach in to exhale fully. The muscles you are using are your abs. Get a a feeling for how they work!

Spanks: Not sure what you mean. Suck in your gut? Isolate your abs? Would focus on core conditioning. This includes both back and abdominal exercises. Doing crunches all day can sometimes make your gut look bigger. Like any other muscle you need to alternate your exercise and work all parts of the abdomen and allow periodic rests couple times a week.