Especially core. I do 100+ pushups a day (in sets), work out 3-4 times a week (run 2-3 mi., some upper body weights, static holds, etc.) but still doesn't seem like my core/stomach area is toned at all. What do I need to add/change?
(I'm not large at all, fairly small-framed, about 5'4"/164cm and close to 125lbs/about 56kg)
If its fat you're trying to loose then cut any fatty acids and unecessary calories out of your diet if it's just defenition you need then there's any number of exercises for your abdominals which I'm positive you know all about anyway.
One thing I would say Is a possibility is creatine, it's a (great) supplement that increases the water retention of your muscles. There's no evidence of side effects from long term usage although for best results do it in cycles (2 weeks on, 2 weeks off) if you do use it you will get muscle gains and increased strength everywhere, it assists building abs quite well I think.
If you do take creatine then do drink a lot of water, a few litres per day. And take the creatine 1 hour before your workout, it takes 1 hour to enter the body and remains in the body for 1.5hours.
what kind of body you trying to get... ex. ryan reynolds, arnold, or some triatlete with abs?
have a specific goal and I can help you more...
1) you cannot target fat so doing 100000 sit up a day will not help you get your abs (actually it will help but only a bit).
You need to monitor calories needed and burned ( you need to be in a caloric deficit to get cut)
To not get in too much detail, an easy way to start off is calculate your daily caloric intake now and try to knock off 200 (if you are already pretty lean) if you are bigger (higher body fat %) knock off 500 calories. then as weeks progress make adjustments. Read up on forums ect. bodybuilding.com is great. not everything there is just for bodybuilders... great stuff on getting cut.