There are no general rules. To some extent, the situation varies country by country. For example, it's illegal to give or receive a tip in Singapore. (That simply results in, for example, the bar staff consistently 'forgetting' to give you your change).
It will also depend upon the type of establishment you're staying in. You can probably leave a smaller tip at Mr's Scroggin's B&B in Cleethorpes (at, perhaps, �25 per night) than you would at The Emirates Palace in Abu Dhabi (around �8000 per night for a suite).
In a moderately-priced UK hotel, a couple of quid would probably keep a porter happy for taking your bags to your room. If you're staying for a few nights, a fiver for the chamber maid might be reasonable (or a tenner if you're staying longer). A tenner would probably be OK for a helpful concierge but if he'd recommended the best restaurant you'd ever been to, given you loads of tourist information and saved you money by calling private hire cars rather than black cabs, �20 might be more appropriate.
Incidentally, not everyone expects (or accepts) tips. I used to run a railway station and we were frequently offered tips. We always refused them. (We weren't barred from accepting them by our employers; we just agreed that it was wrong to expect extra payment for doing our jobs). If customers insisted on giving us something, we'd usually give the tips to charity.
Chris