yes, what jampat said is great. Use MLU (mirror lock up) if you have it. A shutter remote, such as the RC-30N or the like for Canon, is great. If you dont have that, but have bulb mode, which almost all DSLRs have (and SLRs), if just as great. A tripod is a MUST.
here is a shot i did with an EOS-3 a few years back..
i am going to use my G9 and try and see what i can do. I think she has a maximum of a 15 second shutter, as the tiny sensor would get way to hot.
and dont judge me on it not being perfectly square....I was wasted when taking these, lol
I mean it WAS the 4th of July, after all!!