Quick list (with some repeats!)
1) Keep the percentages simple. Don't just change the inches to centimeters. While technically still correct, it assumes that your printer has some reasonable number skills.
2) Make sure you reduce both dimensions. Obvious, I know, but I've seen it happen!
3) Keep the larger size in mind when deciding on image resolution. For example, if you're setting up a document at half size, then double the image resolution. (ie. invert the fraction, so 1/4 the size becomes 4/1 times the resolution). So, if you import a 300dpi scan at 100%, and you want your poster to be enlarged to 200%, the final resolution will be 150dpi.
4) Check with your printer about image resolution - generally, large format prints get away with about 120dpi, sometimes even lower. So don't waste time with 800mb files!
5) Tell your printer what you want the final size to be, and be helpful by telling him the percentage enlargement that relates to this.
Other than that, just the usual 'collect for output' stuff. After you do one job, you'll find it pretty easy!