My rear mech isn't shifting as well as it might.

I'm also broke.

Where's the best place to start looking? I'm thinking that swapping the cables is the first place to start? What's the usual lifespan of a rear mech (I run 2 chains in alternation, not worn one out yet)?

give the cables a good lube then have a look on the park tools site re: proper setting up.

I'm able to set up a rear mech (which is why I know there's something mechanical wrong, rather than just setup), and normally I'd start with new cables (full length outer, so lubing is as more work than putting new ones on), but I'm being a bit cautious as I don't want to buy bits I don't actually need.

What he said – check play in pivots, that's the killer generally.

You can generally work out from the state of the cables of how much friction there is whether they need replacing

Could be cables that need lubricating, bent mech, space to guide pulley not small/large enough, worn cassette, stiff link, sometimes the mech can rotate forwards which ruins shifting… can't think of any more right now.

Rear mechs will generally last a good few years so long as jockey wheels are changed and they're not bashed about too much.

EDIT: beaten to the punch

lubing might be even more important with full length outer – there's more drag – it's cheaper than a new cable plus outer as well.

my rear mechs have lasted for years – normally I replace them when the jockey wheels are knackered though.

you sure it's not the cassette that's worn?

Mech hanger could be bent. If your cables run free and easy with the mech disconnected then they are fine, regardless of how old they are… so then it's mech hanger. They get knocked about prety easily.

If it's a full length cable run though, it's likely to be the problem. On my 5 they are a bane – I am tempted to consider not lubing them at all, becuase I think the lube at the mech end attracts gunk that ends up in the cable.

Bent mech hanger-you will NOT be able to eyeball it-you need an alighnent tool (or a bodged alternate).

I'll pull the cable and blast it through with whatever aerosol I have with two letters and two numbers.

I'll look at alignment etc. too.

Do jockey wheels wear and affect shifting or are we talking about really worn out before they're dead?

Cassette etc. seems fairly good – whole drivetrain is of the same age, so it's not been through 2 chains yet, if you see what I mean.

Pull the cable outer end caps off and trim the snaggly bits from the outers

What make/model mech is it?

Shimano Deore from around 2005. I'm pretty fastidious about chain cleaning etc. so it's possible that the mech has worn out before the rest, but I did bugger all riding for 3 or 4 years.

Check the top jockey wheel isn't gummed up with cack – if it isn't sliding smoothly on it's sleeve then shifting can become a bit hit and miss. Also check the lower jockey wheel is smooth while you're there. Check that the main spring around the attachment bolt is smooth and the mech is maintaining tension in the chain. Beyond that, there isn't really anything else you can do to "service" the mech, so if it's not something to do with the cables/hanger/alignment as the others have said then it might be new mech time.