In customizing, we're going to do a great deal of cutting, so we need to have the right tools. Your primary weapon is going to be the Exacto Knife. The one pictured in (A) has the blade built into the handle, giving it greater strength and control. However, blades dull so the one you'll want to get is (B) which can be found at Wal-Mart, Michaels, or hobby stores and usually come with replacement blades (C), or you can buy sets of blades for different tasks. The blades are held in place with teeth that tighten with a twist at the top of the handle, making changing blades pretty easy.
Our next cutting tool is the trust Snips (D). These are the general kind you might find at Lowes or in the hardware aisle at Wal-Mart. A more delicate, precise kind for snipping model parts off of trees can be bought from Testors or a hobby store, but are usually more expensive. These are good for stripping wires, cutting paper clips, or trimming off hard plastic pieces. And basically, if you're using these, you should be wearing eye protection.
Basic utility scissors (E) are also nice to have in our toolbox. Most kinds come with a handy gripping edge in the middle for opening bottles, but has other good uses.
One of the best attachments you'll find for your Dremel are cut-off disks (F). These come in different types for cutting either metal, wood, or plastic. Generally, whatever is suited for wood will work for plastic. These are relatively inexpensive considering the many uses they have. I probably use these for 75% of the cutting I do and they last a long time.
Finally, when you need to cut something dense, big, or thick consider a hack saw (G) or hobby razor saw. The teeth on a hobby razor saw are finer for cutting plastics, whereas a hack saw is likely to gash plastic. But it's useful for cutting wood or metal.