a lot of good advice above. here's mine.
-diet plays a huge part. try eating more whole foods and less processed food. try incorporating some good fat into your diet.
-don't over eat. it's better to eat more small meals than cram as much food as you can fit in fewer sittings.
-core exercise is very important for bike riding, and you can almost never over work your core. try adding core exercise to every day things like watching tv. during commercial breaks do sit ups or plank for the duration of one commercial. as you get comfortable, do them for another commercial. keep in mind that if you get to the point that you can hold plank for an entire commercial break, you're a disgusting freak of nature and people wont want to talk to you.
-my final bit of advice is to listen to Peyton Manning and accept that unless you're in your early 20's and a professional athlete, you don't need six pack abs. i'm a human stick figure, so i had a defined stomach up until my mid 20's. i'm 29 now, and while my stomach is still flat, i've got a nice layer of blub covering what is still a pretty strong core. it's just life, and having some body fat isn't so bad. I can produce ear wax now, and i couldn't when i was 3% body fat. that led to a ton of ear infections. bet you never thought about that, huh?

i don't know much about your diet or age, so all of that up there is generalized and what works for me.