How to approach a professor for research?
How should I contact a professor about joining his/her lab? 


Email now. Express interest and see if they're willing to talk via Skype or would be willing to meet in person when you're both in the same city. You want to talk face to face. This is also a good spot to ask if they have any advice on what to read to get up to speed on their work if it's going to be a while until you can meet them.


  Do professors generally just look at resumes, or do they expect prior research experience? 


No to both. Research experience is a plus, but you have to start somewhere.


  Should I indicate the topics I'm interested in?


You should be interested in the topics that the professor works on and should identify what those are in your initial email.


  I haven't decided about whether I'd like to do a PhD, so does my
  indecision make it harder for me to get a position at a lab for the
  1-2 years it'll take for my Masters program?


If you end up wanting to do a PhD you should go somewhere other that where you did your Masters and undergrad work. Any decent professor shouldn't expect you to stick around. 

There are some professors who won't work with Masters students. There's nothing you can do about that. Others will want you to take their class first. Take it, ace it, and then remind them that you're still interested and now have other ideas from their interesting and wonderfully taught class.

Now for Ric's guide for getting into research


Go to your department's website and find the page listing all professors (hopefully with links to their websites).
Skim each professor's bio and see if their research areas interest you.
For every professor that sounds interesting, read through some of their current papers. Focus on more recent works. Read abstracts and intros. Use this to get a better idea of what they're working on. 
For every professor you're still interested in, find a recent paper or two that you like and read it fully. In this step you want to try and figure out what they did, why they did it, and how their work fits into the current work in the field. This is the hard part, but the more effort you put in here, the better. Besides, you want to get into research, right? You need to learn to read papers =)
Send a short email to each professor. Something like:



  Professor Webb,
  
  My name is Francis Thurston and I am a first year Masters student interested in meta-physical topology. I was reading through your recent paper on non-Euclidean architecture ("His House at R'lyeh: Modern architecture of Greenland cults") and was wondering if you had considered Professor Atwood's claims on the subject ("Mountains of Madness: The innate wrongness of the city") in light of your recent work. I think that combining his claims with some of the results of your recent experiments would be interesting. Would it be possible for me to come by your office next week to discuss your work and any possible research opportunities?
  
  Thank you,
  
  Francis Thurston



When you meet with a professor. Ask questions that show you're interested in the subject and have put in some work. Extra points if you have ideas about directions their work might go in (e.g. areas to explore, experiments to run, &amp;c.). Ask if they're taking students, if they would consider you, and what the workload would be like.