Just wait, Iain! Hold on, if you can! In less than two months, Halo: ODST will ship which has an entirely new campaign (you play as an ODST). It also comes with a separate disc which contains everything you need to play Halo 3 online - and by that, I mean it includes all the map packs, including three maps that aren't even out yet (they're being released with ODST). You can order it for £30, which represents excellent value for money if you don't have the other map packs already.
As for improving, well, it's tough. Halo has been out for nearly two years now, and the people still playing it are mostly loyal fans who've had it since day one. The other thing to realise, too, is that many people now have second accounts, or "boost" accounts. This simply involves high-rank players setting up another gamertag, so that these experienced players can play low-level games again, or help their friends progress up the ranks (boost).
Some very basic tips:
1. Learn to use the BR (battle rifle). It's the go-to weapon of any serious Halo player. Why? Because a single headshot with a BR on an enemy with no shields will give you a kill. This leads me to the next point...
2. Practice using your grenades. Getting "sticks" with plasma/spike grenades is fun, but if you have a BR (or a Carbine, it works similarly), then any well-placed grenade will take off even a full shield, allowing you to pick up an easy BR kill with the headshot.
3. Learn the maps. This includes weapon spawns - you need to know where the power weapons are. If your team doesn't have them, avoid these areas. If the power weapons are respawning, grab them as soon as you can. Don't forget, too, that weapon respawns are timed - keep an eye on the clock in the bottom left of your screen. They normally reappear every two minutes or every three minutes. The same goes for vehicle respawns and power-ups/equipment - anything you can pick up and use has a respawn time. Avoid "busy" areas on the map, especially in Lone Wolves, and try to pick off kills from weakened players in these areas from a safe distance.
4. Learn from others. Use the the theatre mode - watch back games and follow the best player. Check what weapons they were using, and when. What routes did they take around the map? When they died, where did they respawn, and what did they do?
5. Try customs! It's a great way to practice new tricks and tactics without the stress of matchmaking.
5. Practice, practice, practice! It's easy to get disheartened, but try not to. You'll get there in the end.