One of my biggest concerns as a developer (and I have quite a few) is being able to stay relevant. This doesn't necessarily mean making myself the only go-to-guy for a project or getting upset when I'm not invited to meetings but staying relevant as an experienced developer.
So to stay relevant what do I need to do?
Well the first thing is to keep my existing skill-set up-to-date, calling myself a .NET developer is all well and good but if I only know about version 1.1 and not 2, 3, 3.5 and 4 then how useful am I and how am I able to help influence technical direction if I don't know about what's new!
Next is a harsh one, but necessary. Know when to move on from an existing skill. I know we all feel comfortable with what we know but if you're a C++ programmer and there are no C++ based programmes left to maintain or write then should you spend as much - if any - time investing in those skills. I'm not saying forget about them, and from time-to-time it's nice to come back and brush up a little but sticking with it as a core skill means you'll be slowly phased out like the programmes you maintain.
Try to keep up-to-date with new theories and practices. Some times people do re-invent the wheel and sometimes it's a good thing, maybe it's a new design pattern or a new way of looking at threading; but knowing about these can help make you a better programmer.
Keep your eye on the horizon. Sounds a little managerial I know but looking at what's coming up is really useful as it will help to figure out where you should be spending your time. Maybe looking at a new language instead of an entrenched one will help with a new product or problem that you know is coming up, or maybe it might just be more fun.
Enjoy what you do. Sounds obvious but you go to work every day and churn out code without really enjoying it you wont have the motivation to spend the time learning new things and before you know it you're out of touch and out of date. This can be a tricky one though, if the project you're on isn't that interesting then how can you stay enthused about it? Well look for little things around the project you can do in your spare time to make it more interesting, such as writing a little app to make a repetative task more efficient. Contribute to an open-source project you like or just write a little app for yourself, some of the best applications have been written to scratch your own itch.
The last thing is a tricky one for some people but here goes. You DO NOT know it all, you might have at some point but things move on, and quickly so you will need to as well. But there are people at the other end of this scale and to those people I say you DO know something, there is no such thing as a perpetual noob, every day you learn something you're more experienced than the day before. Look at it this way as well, even people who write programming languages don't know every little aspect of it as other people contribute ideas and write libraries and frameworks and they don't know how all of them work!