1. Upgrade the photographer. 2. Upgrade the support and shot discipline. 3. Upgrade the lens. 4. Upgrade your camera.
You should read the whole article for the full explanation of all of these, but you get the gist.
In general, I'm with John on this one: why do you need to crop? Is it because there was no way possible to get closer to your subject? In that case you might want to think about a longer lens - and possibly trade in some lenses you're not using. The other possibility is that you just need to get closer before you shoot. We can't tell you that without know what you're trying to get a picture of.