1: Avoid the crappy supermarket fuel where possible.
2: Use Super Unleaded from one of the big fuel companies, like Shell, or BP, you'll find it'll keep better, and give you less problems.
3: For use in two stroke machines, it advisable to use up the fuel and replace with fresh after 30 days. This is a guideline most machinery manufacturers are recommending.
4: Don't use fuel left over and sitting in the can from the previous year, it really won't do you any favours, and might cost you big time to have your machine repaired.
5: Don't leave fuel sitting it the machine all year round, or if you do at least start it up a few times during, so it replenishes the fuel in the carburettor. Ethanol is at it's worst when sitting still in your carb.
6: Pump fuel in the UK contains 5% Ethanol, not only is it corrosive, it's also hydroscopic, which means it'll corrode the aluminium your carb is made with, and it also draws in moisture from the air, so you'll eventually get water damage. Hence why a lot of mower carbs are now made from plastic.
Alkylate based fuels. (IE: Aspen, or Stihl MotoPlus). *
Keeps for around five years in it's original coloured container, but keep it out of sunlight, and in a cool place. If you store it in your clear strimmer tank which lets the light in it won't keep for as long.
Alkylate based fuel doesn't contain the nasty Ethanol. so that eliminates 99% of your problems, it also doesn't contain some of the other nasty chemicals which makes it better on your health.
* Why didn't I include Stihl MotoMix above?
Well as far as I can make out, (And someone will no doubt correct me on this), Stihl MotoPlus (neat fuel for 4stroke engines) has the word Alkylate on the can, while Stihl MotoMix (Premixed two stroke fuel for two stroke engines) doesn't, and from the data it states it's a fully synthetic fuel, which suggests it's completely man made.
MotoMix again has a five year shelf life if stored in the original container, but they also state it will stay fresh for extended periods in your equipment, though how long I haven't a clue. I'm guessing around two years, based on the fact that the early 1Litre bottles of MotoMix were shipped in clear containers, and had a two year shelf life apposed to the five litre coloured can which had a five year shelf life.
Note: I would also recommend, anyone who's been running chainsaws, strimmer's, blower's or anything which uses a diaphragm type carburettor on pump fuel for quite a while, change the diaphragms before switching to Alkylate based fuel, it's pretty common for the change in fuel to make the diaphragms go hard, and therefore give you running problems.
Anyway, hope you find this useful, and remember, keep it lean, and keep it fresh, it'll save you money in the long run.
Information above is my own thoughts and observations, if anyone would like to add or comment, then feel free to join in.