A pair of pliers with a rubber band around the handle makes a great motor clamp. Lay the motor inside the handles then wrap the rubberband around the motor & pliers.

If you have any flux add a little to the motor tab. It'll take a decent amount of heat to get the solder to flow onto it. I solder A LOT where I work & if there is one tip I would give to anyone getting ready to fire up an iron it would be WEAR SAFTEY GLASSES!!! I know it sounds corny & it doesn't seem like that big of a deal but I've felt the sting of solder on my face more times than I'm comfortable with & I have picked plenty of solder off my glasses. Of course you could go without them & take your chances. I don't want to use a lot of gory details but I will say that if a trip to the hospital, a magnetized drill bit, & your eye sounds like a good combination then take your chances. I've soldered many motor & battery wires over the past 14 years. It only takes a little bit of pressure for 14 gauge wire to make a good spring.