After doing some stretching I usually do these before a workout, without stopping..
Roman Chair Crunches x 50
Leg Raises x 25
Knee-Ups x 25
Reverse Crunches x 25
Hanging Side Twists x 20 each side
Leg Raises x 25
Crunches x 25
Standing Twists (for obliques) x 50 each side
Stomach Vacuum x 1 minute
Then I'll do some more ab work at the end of my workout. Maybe another multi-set thing like this. I find frequent exercising of the abs works best for me as long as I don't overdo it in one day. When I feel like I need to rest them, I do. But I'll go something like 3-4 days, then rest a day, then 3-4 more, rest a day, ect..
Don't really do a lot of weighted ab work. If you use weight, you're likely to make the muscle bigger. If you want a slimmer waist, don't use weight. That way, they'll look tight but not big.