Photoshop CS2 has preset document sizes that match common video frame sizes and pixel aspect ratios, and they include guides that match the video safe margins.
You can also created new guides on an existing document by using View > New Guide, and entering a percentage value. The standard sizes for safe guides are 10% and 20%. (For Photoshop, since you'll have to create guides for each side and each orientation, that's 8 guides total: 10% horiz. and vert., 20% h. and v., 80% h. and v., and 90% h. and v.)