Tuk-Tuk fares
Fares vary, depending on the distance travelled, the time of the day, the traffic, and the mood of the drivers. Normally a very short trip will cost 30 baht.
Remember to bargain
Fare negotiating and haggling is a must because the price named by the driver is always an 'inflated rate' (especially if you're a tourist). The trick is to negotiate 5 - 15 baht off the proposed fare, and take it from there.
Beware of scams
Be careful of the 'mafia' Tuk-Tuks around touristy areas, who often boasts privileged knowledge of 'secret' or 'special' shopping places and things. Some of them may offer sightseeing tours and unsolicited help to take you places. A short and sweet "no, thanks" will save you from their scams. The same rule applies to taxis.
Avoid the peak hours
Avoid taking a Tuk-Tuk during peak hours (07:00 - 09:00, 16:00 - 19:00). You don't want to be stuck in traffic for hours, sweating and breathing in the hazardous fumes from engines all around you.
Tuk-Tuks are faster than taxis
Tuk-Tuks are most ideal for short trips. Sometimes it would cost the same, or even cheaper, to take a cab to the same destination, but it will go a lot faster.