I design most everything at 10% of final size. If it's something that will be looked at from fairly close I use 720 to 1500 dpi. If it's something to be seen from pretty far (billboards, mesh banner) I usually go between 500 and 350 dpi.
When you send it to the rip you then increase it by 1000% and you get your final size and appropriate resolution.
Of course, offset is a different beast all together. That I always lay it out at 1:1 with a minimum of 450 dpi
Rule of thumb: You want the printer resolution to be at least 4 times the image resolution. For example, printing at 720dpi you'd want to have an image at ~150ppi. The 4:1 ratio typically yields the optimum printer pixels per image pixel. Something on the order of x^16 where x is the number of colors you're using plus 1, as in 4 for CMYK plus 1 for the white media. A ratio larger than that accomplishes nothing. Less than that can affect color fidelity.
The unaided human eye is incapable of seeing any sort pixelization at any image resolution more that ~130 ppi. Images with a resolution greater than 150ppi are usually a waste of time and space.
100 PPI (not DPI) at 100% should work fine for most anything. If doing fine art I like to use at least 140 PPI. Really depends on how good the image is to start with, your rip, your printer and what your printing on.