Thought my drum fueling days were over, but alas . . .
If you are using a hand pump, the most important thing to remember is you may need to wind the handle quickly, initially, to prime the pump. In some cases (especially when a drum is half-empty and gravity is not assisting as much as it does with a full tank), winding slowly may not prime and hence you may think the pump is broken when it's not.
Make sure to wind in the right direction! (Usually clockwise)
Many hand pumps deliver a standard 1 litre per turn – check the pump to see how much each turn delivers so you can calculate how much to pump into each wing (saves having to remove the spike to check the barrel with a torch). When using an electric pump, time how long it takes to empty half a drum (or whatever volume) so you have a reference point next time for how fast it fills.
There is talk of putting blocks under bungs to prevent any water/scale/rust from being pumped from the bottom of the drum in the first instance. It would be much better to simply pump the first litre or two into bucket for inspection before you start refuelling the plane. If you don't have a bucket, pump a couple of litres on to the ground and observe the colour as you're pumping. The pump should also have an inspection window near the filter so you can see the colour.
Drums should be sealed and dated, and in good condition (no dents, especially near the seams) – if not, be wary.
Carry a pair of pliers with you in case the drum wrench goes missing – you can use them to remove the seal and open each bung.