OK, I know some of the posters here won't agree with this but I"m actually qualified for the middle-aged grouping so I'll give my 2 cents:
Of course diet is important. I think as you get older, you don't need quite as many calories as you did before. It took me a while to realize this, but in my own case, I'm actually doing better on less carbs a day as long as they're low GI index (except post workout.) Everyone's different though. It takes an increased amount of awareness and a bit of experimentation to see if some fine-tuning is in order.
On the training side, I'd have to say there's two things that have proved useful. The following is strictly from what I've noticed in my own training so take it for what you think it's worth. First, change your emphasis from 'working abs' to strengthening your core. In a sense it's almost the same because the core focuses on your abs, but you're now more concerned with strengthening the entire support system for the rest of your frame. You're now attacking the problem from a relatively unused pathway that will also improve the rest of your training as well.
Torso Training for Athletes is the first of a 3 part series that explains the kind of stuff you'd be doing.
The second thing is periodization. In 2-3 weeks your body adapts to the stress you're putting on it. You need to rotate exercises frequently to avoid this. For abs (core) especially, I use a pool of 4-5 exercises I find effective and change things up whenever, even from week to week. The Weider intuitive training principle at work.
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