I use a mirror that is attached to the bottom part of my handlebars. I can see a long distance behind me without having to move. If I am going to turn left, I still look quickly over my shoulder.

I like this set up as it is easily adjusted and is larger than the glasses type mirrors.

Re adjusting your seat- yes, get a multipurpose tool. It will have allen wrenches, a screw driver and other handy tools you might need on the road. I've adjusted my seat many times using this tool.