1. Select the The Best Equipment for the Job.
Although it might seem obvious, you should make sure that your toolkit is customized for the bike you ride.
In order to do this, you must bring the right size axle and spoke wrenches, the correct size sockets, hex keys, and torx bits, as well as a patch or plug kit depending on whether your bike has tube- or tubeless-style tires.
Finding any specialized equipment that may be required can also be very helpful; for instance, I am aware of a few bolts on the R1200GS have that strange T37 Torx bolt in there... Perhaps, if you ride an Harley or Triumph, you should get that SAE (Imperial) set out. Not every garage or roadside help has those...
2. Keep it simple and light
Even though it's crucial to make sure your tool kit has everything you need to work on your bike, you don't necessarily need to carry a full Snap-On toolbox around with you.
The less separate parts you have in your tool bag or roll, the better. Utilize multi-tools, such as folding hex key/torx sets (my personal favorite), and combo-tools, such tire levers with an axle wrench on one end, to streamline and consolidate your tool kit.
One you cannot miss is a Leatherman or other brand of combi tool. Those just rock!
3. MacGuyer it
You might have broken your brake lever or discovered a hole in your clutch cover after dropping your bike on some rocks, in which case a wrench or a screwdriver just won't be able to aid you much.
It's time to turn up your DIY engine and get the bailing wire, zip ties, and JB Weld ready. A couple zip ties could save your ass while riding a bike.
4. Tyre Kit
It is easy to get a puncture and they will never happen when there are people, garages etc around. So get a spare innertube and a DIY patch kit. You ride without!
Got any other tips? Share them and we will add them for the rest :)