|Art by Wayne Reynolds|
Re-skinning is one of the most useful tools at a GM's disposal. It allows you to create something new and interesting without creating an entirely new mechanical structure to represent it. All you have to do is pick something that already exists in the game and change its appearance.
Your players are exploring a crashed spaceship and you want them to find some pieces of advanced technology? Just re-skin some magical items with a technological visage. That wand of magic missile becomes a laser pistol and the magical charges become the power cells. That fancy laser sword the fighter found is actually a +1 brilliant energy longsword. It's that easy.
With that being said, you should still put some forethought into the re-skinning process. Since you will be using an existing mechanic to represent your new idea, you need to figure out how that mechanic will be implemented after the change. For example, let's say you are planning to re-skin a couple of kobolds into a hive of bug-men. Knowing that kobolds receive a bonus to their AC due to their scaly skin, you decide that bonus is a representation of the bug-men's exoskeleton.
Also, you should think about the problems that might arise from re-skinning something. Let's look back at the wand of magic missile that has been re-skinned as a laser pistol. Since its now a piece of technology, does that mean its immune to effects that would normal screw with it like spell resistance? If you say yes, does that make this piece of technology overpowered? If you say no, why is this piece of technology hindered by this ability that effects magic? Does that mean magic and technology are similar? Just take some time to think about the changes you are making and the questions that might come up because of it.
However, don't let the problems stop you from utilizing this useful GM trick. A simple re-skinning can take something boring or average and make it interesting and unique (flavor-wise at least). Just be careful with how you do it and give some thought to the repercussions of the changes you are planning to make and you should be fine.