Now for mastering, the key here is to not have many peaks before you export your mix. Having wave form preview turned on, you should be able to determine where your peaks may be coming from. Again, sometimes eq is the problem on the instrument. It's amazing what you can control with eq these days. But if you have to compress a bit, just make sure you are using the right compressor for the right reasons.
But in mastering, I stopped using the compressor I used for the longest time. The reason being...my belief is I've learned a lot more about how a mix should be before I export it and I use a 2 bus compressor now. I never used to use one, but the key there is having the right one. I'm actually astonished at how much it's made a difference for me. Everything I've used it on sounds incredible. Everything I don't use it on...seems to be lacking.
That said, the 2-bus comp glues the mix together and also forces you to make eq changes that you wouldn't make if it weren't on the master bus. It's not a necessity, but has made a difference for me. Due to this, when I master, I rarely use a compressor on my own stuff. For clients, sometimes I use a compressor (Waves API 2500) but most of the time I find myself getting the best results using my UAD Precision Multiband compressor. Used lighly, the way I'd use my compressor while mastering, this also gives me frequency control which is a nice plus. You don't need much, that's for sure. And then of course I limit at the end of the chain with either a Waves L2/L3, PSP Xenon or UADF Maximizer. They all have different strengths for different styles of music.
But that said, before I even get to the mastering stage, I like to control peaks manually. By using a program that can literally allow you to raise/lower individual peaks by zooming in on them, (Adobe Audition is excellent for this) it allows you to just concentrate on any peaks that may be a problem. The highest peak in the file will be your loudest point if you just put a limiter on. If you control that peak and lower it, you can actually make the song louder IF need be. Though it takes a long time to edit peaks if they are in excess, it's a fool proof way to manually edit your audio so that you don't over limit while getting artifacts. Anyway, hope some of this helps. :)
-Danny