I have a lot of memory on my sd card but my phone memory is getting full
With the coming 2.2 update, you'll be able to install apps to your sd card...if the app developer allows it.
You can also go through your apps via phone settings > applications and delete their cache. Sometimes an app's cache can grow to surprising levels. For example, the game cestos is simple and not a large app but it's cache grows really huge. There may be apps in the market for automatically clearing caches.