Let's make sure we're talking about the same thing.
Android isn't Windows - it tries to keep as much in RAM as possible at all times, so no free RAM is a good thing. Android has a great memory manager - if it needs RAM for another app, it'll force kill an app that hasn't been used in a while. But 512MB of RAM is really small for most of today's apps. Some of them won't run at all in so little space. And trying to run more than one at a time is going to be a problem. (No, you can't add RAM to a phone, you replace the phone.)
If you're talking about 0 free storage (also called ROM), that's another thing. That just means that you're putting more apps and data in the phone than it has space for. You can't carry a gallon of milk in a quart container. The only solution is less milk or a larger container - IOW, either fewer apps (or move all your pictures, videos, music, etc., to an external SD card) or a phone with more storage. (4GB of storage, part of which is used by Android itself, is hardly enough these days.)