For me all starts with the right palette. It really helps to get what you're looking for. I usually work with 64 colors (unless there's any special restriction, of course).
For me it really helped to configure Gimp for pixel art: set the pen to 1 pixel, save some palettes, set the grid to 8x8 when doing Spectrum graphics, a keyboard shortcut to show/hide the grid, etc. Gimp defaults are not good for pixel art.
Other than that, jut zoom as needed. I usually scale the games x3 when trying a retro style, but in Gimp I draw mostly at x8.
It's pretty ancient, but here's Larry Ewing's notes on how he created the original Tux image in Gimp: http://isc.tamu.edu/~lewing/linux/notes.html