Get a plastic sandwich baggie and put some Vaseline in it. Then put your penis in and squish all the Vaseline around so it covers your penis. Once that is done, kneel in front of a bed. Lift the up mattress and put your penis, with the baggie still on it, between the mattress and the box-spring. Start pumping as if you were having sex. When you're done, all you have to do is throw away the baggie.
BOXERS & MATTRESS
Get two pillows and sandwich them under the mattress, wrap a pair of boxer shorts (or something else soft) around your penis, and fold a Kleenex into the end, over the tip. Then insert your penis between the pillows and pump away.
BUBBLE-WRAP & MATTRESS
Instead of using a baggie, use a piece of bubble-wrap — the plastic stuff used to wrap fragile items for shipment.
BAGGIE & MATTRESS & SOCK
After putting your lubed-up penis in the baggie, unroll a heavy sweatsock over the whole thing, like a condom. The sock shouldn't have too much left over at the base of your penis, though, as it can limit your range of motion. Then put the whole thing between the mattress and box spring, or under a chair cushion, and go for it! The sock reduces possibly uncomfortable friction while still hugging your penis.
HEAT IT UP
Put some lube in a sandwich bag; then get a heating pad and fold it in half. Set the bag of lube in between where you folded the pad in half. Place the heating pad (still folded in half and turned on a low or medium setting — NOT high), with the bag of lube in it, under a couch cushion seat or between your bed's mattress and box spring, and insert your penis into the bag and proceed like you're having sex.
ADD A ROLL
Get a condom, a freezer baggie, a toilet-paper roll (with no paper on it), two pillows, a bed, and some lotion. First, lube up the baggie with the lotion and spread it all around inside. Next, feed the baggie through the toilet-paper roll and spread out the open end. Get two pillows and put them between the mattress and the box-spring, and then put the toilet-paper roll with baggie in between the pillows (open end out). Put on the condom and lube that up, too. Insert your penis into the open end of the toilet-paper roll, and pump away. The toilet-paper roll gives you that tight squeeze, and the condom prevents that uncomfortable rub from the plastic baggie.