Legal bit:
Hirer insurance is third party and if involved in an accident you are responsible for damages.
Thailand requires a IDP if you do not posses the new style photo ID driving license. You can obtain a IDP from the AA or RAC.
Emergency vehicles carry flashing amber and red lights. Royal Thai Highway Police vehicles are either brown or maroon with white doors and Police vehicles are all blue or all white.
If a Police officer blows one long whistle you must stop. Two short blasts means go. Security guards, carpark attendants, and road workers will also use a whistle. If a Police Officer holds up his hand facing you you must stop or if he has his back to you and is holding up either hand.
Lane position:
Drive on the left. On dual carriageways use the left hand lane first.
Most roads have a hard shoulder that is an unrestricted lane and may be used to stop if you have a problem but do not park, it may also be used as a shortcut by motorcyclists passing in the opposite direction.
A motorcycle travelling the wrong way has the right of way.
On single carriageways all oncoming vehicles have the right of way. If an oncoming vehicle is overtaking manouvre to the hard shoulder to give him room. If there is no hard shoulder move as far to the left as you safely can.
Lane markings:
A single solid white line on single carriageways denotes the centre of the road.
Single or double yellow lines means do not cross the lanes.
Short broken white lines means you can overtake (Do not overtake when there is oncoming HGV - they cannot give way to you).
Do not enter white hatched or chevroned areas. Yellow hatched means no stopping or waiting.
Junctions are marked which lane is for what exit.
At traffic lights or trunk route filter lanes do not use the left lane to go straight on.
Road signs:
Major road information signs are green and include the highway number. Some give advance information for trunk route.
Minor road information signs are blue and include the phonetic spelling, direction, and distance.
Local information signs are white
Restrictions are white with a red border and round or a circle on a white background.
Warning signs are yellow diamonds with a logo depicting the hazard ahead.
Instruction signs are blue with a white border and round or a circle on a blue background
Trunk route markers are a white square with the route number. If the route marker is at a junction there will also be a white square with an arrow depicting the direction.
Junctions:
Traffic lights are Red Amber Green.
Sometimes there is a counter that tells you how many seconds to the next light change. Do not go before the clock gets to zero. If the light is on green do not try to cross if there is insufficient time.
If the light is on red you may turn left but you must give way to traffic from your right.
If on green you are turning right move to the centre of the junction and give way to oncoming traffic.
When oncoming vehicles are both turning right they pass in front of each other.
If on green you are turning left you must give way to oncoming motorcycles.
If the light is flashing red you must use the crossing as an uncontrolled junction. You must stop and give way.
If the light is flashing amber you have the right of way but must slow down and look both ways.
At roundabouts give way to the right.
Do not shortcut traffic islands and signal your intended manouvre.
Sometimes roundabouts have a sign for a U-Turn. This means you may turn right at the roundabout if you need to travel back along the road you are on.
Rural Roads:
In very quiet areas take extra care at night especially during national holidays. Be aware of agricultural equipment being driven slowly and farm animals being moved or escaped animals.
Restrictions:
Motorcyclists must wear a crash helmet and must have lights on at all times. Motorists must wear a seat belt. 
The national speed limit is 80kph and inside city limits is 50kph except where signs say otherwise and this includes villages.
Kerb stones painted alternate black and white mean no parking. Red and white means parking restrictions apply. These markings may also be on lampposts or other posts on the side of the road to which they apply.
Insurance is voluntary. Not having insurance is high risk.
For more info on legalities try freebeerforyorky.com