I suppose if you genuinely don't know who to vote for you're a bit stuck. I've never been in that situation, but if I was, I'd probably either vote for the party currently in power if I was happy with the way things are going, or for any other non-extremist party if I wasn't. If you vote against the government then you can turn around and have every right to protest against any of their policies; after all, you didn't vote for them.
I know exactly what you mean about the parties not bothering to campaign at all in areas where they know they won't get any votes; I used to live in a dodgy run-down estate in the corner of Southampton which just happened to be part of the Romsey consituency (Romsey is a leafy market town next to Southampton). The MP was killed in a house fire, but I didn't even realise he had been my MP until the campaigning started for the by-election. I think I got one leaftet from Labour, none from the Conservatives and about thirty from the Liberal Democrats; I also spoke to the Lib-Dem candidate who was canvassing as I came back from work one day. In a famous victory she overturned a big Tory majority to win the seat.
Rich :¬)