A few more details? Still or running water? Soft weed or tough? "Proper" plants or blanket weed over the bottom?
If tough weed, like lilies, fish safety is a big issue- you'll need gear able to bully your quarry out of trouble, set up to fail-safe if you "misunderestimate" them so you don't leave a fish tethered to a root.
If soft, it's just a presentation problem. If you're float fishing, try using a pole to lower the rig in, rather than casting; if legering, pva bags may enable you to crash the rig through all in one lump. For blanket weed, try using a slow-sinking bait like bread-flake.
