Buy a good quality chain and look after it. Then, change the chain and cassette at the same time when they wear. (and probably your most used chainring!) (EDIT: I recently bought a ridiculously expensive chain checker and intend to replace my chains religiously so I will find out if that is a Good Idea)
Try never to put a new chain on old cogs as it will slip or suck. (EDIT: very old cogs)
Steel grannies last much longer than aluminium and weigh very little more. (If you are really tight, you can flip them over when they get worn!)
If you are not too fashion conscious you can also buy long lasting steel middle rings (Deore)