Something to remind you.
Seen in a rental car in Ireland, a sticker on the wind screen reminding the driver to drive on the left.
If you do rent a car without such a sticker, or take a car to where the driving is on the other side of the road, you can make your own 'sticker'.
My brother used a plaster (like you put on your finger when you cut yourself) in the spot where you would look when driving on the right but not when driving on the left.
It has to be just out of your normal field of view when driving where you are suposed to drive, but anoyingly in view when you drive on the wrong side.
Tips for getting started.
Most people prefer driving on motorways to get used to driving on the other side of the road, as most cars go the same direction, you 'only' have to get used to cars merging from the other side and exits (off ramps) on the 'wrong' side and you might select to go to the slowest lane before your exit comes up, or just stay in the slow lane while adjusting.
Have a box talk to you (GPS-Satnav-TomTom-Garmin), telling where to go, so you do not need to worry about where to go.
Set the machine to the system you are used to (American English if you are from America) but to the distance system of the area, (so mile where miles are used but km when in a km country.)
That way you will get the words you are used to but the distances which are also on the signs, and likely on the dashboard of a rental car. If you drive your own you should consider what is easiest for you.
If you have a passenger, get him or her to remind you to keep left when you drive away from being parked. Yes, it will get anoying, but driving away from parking is one of the points where it is easy to go wrong, specially early in the day and when you do get tired.
If you travel alone, you might program your phone to give an alarm with a 'keep left' as ringtone for the alarm, at the time you expect to start your driving. (Make sure it does not need action, in case it goes off while you have already started.)
Other moments you need to be very careful.
When turning one direction a few times in a row, you are more likely to end up in the familiar location on the road. So tell yourself 'keep left' every time you turn a corner.
This is even more likely when you are lost and looking where you need to go. (Satnav-GPS helps avoiding this.)
When driving on small roads, where you will usually be in the middle of the road when there are no other cars, you run a big risk of diving to the wrong side of the road when someone else shows up.
You will have to adjust to the road but it might be better to be a bit off the ideal driving line just to keep you reminding yourself to stay on the proper side of the road.
Go slower than you would normally do on a road like that, and that is also slower than the locals do.
Keep yourself as aware of your surrounding as you can.
As mentioned in one of the other answers, do not drink alcohol before driving, even when the local laws allow you a glass or two.
If you like to hit the bottle/can, you might even feel the need not to drink much in the evenings after driving, so you do not have a sore head in the morning.
But also make sure you do take more rest than you would usually do, (even more than truck drivers need to take by law) so you stay from getting tired.
Have your drinks and snacks handy (or have your passenger hand them to you) and take proper breaks for anything bigger, for hot coffee, and all other things you need to do.
Rest stops might not be where you expect them to be, like in Europe they are often on the side of the motorway with there own entrance and exit, in the UK they are often just of a normal road exit, on a roundabout. So you need to be more aware what to look for when you need a rest stop.
Or they might be spaced different from what you are used to, in low traffic countries there might not be petrol points with restaurants for a few hours, in heavy traffic areas they might be every half an hour or more often.
The old saw of 'use it when you see it, it might not be there when you need it' might be the rule to go by.
If you keep aware what you are doing and go with the flow, it should not be hard to learn.
I do not drive, but have ridden a recumbent trike in the UK (from the Netherlands) and I have been a passenger/navigator with many friends and relatives going across to the UK or from the UK where you need to drive on the Right.