This the solder splice that offers the least resistance I've ever found : less than a hundred micro ohms! My Ohmmeter doesn't read less than that, so I'm happy with it.
It's quite easy to do and offers great mechanical resistance for virtually no electrical resistance :)
Step 1: Wire Wrapping
First you want to remove a good 5 cm (2 inches) of insulant on the wires, then twist them on themselves to make a single thick brand.
There's a great scheme here that came from wikipedia and sums up the following:
1) Fold in the middle and give one or two turns
2) Wrap tightly around the other wire using pliers or your fingers if you feel strong today
Step 2: Drown It in Solder
Finally, heat the wires with the tip of the soldering iron and then apply the solder by touching the wires with your soldering wire.
Once the whole thing is soaked in solder, you're done!
You can easily slide on a piece of heatshrink and the splice should withstand the strongest of tugs while guaranteeing incredicle electrical contact.