Just know ahead of time that the roomettes are smaaallllll. Sometimes when I try to explain this to others, they will respond by saying, "Oh, so the mattress is like a twin sized bed?" Not even close. I'm not going to bother looking up the exact dimensions but the entire roomette is probably smaller than the length and width of a twin size bed.
My point is, if you haven't seen one of these rooms, don't expect anything too spacious. But for one person, I think a roomette is perfect. The fact that you get to lie flat and shut the door at night is what more than justifies paying the upgrade for a roomette. Beats coach any day.
Hmm other tips...
Bring all the toiletries you'll need. They generally provide bar soap but there's no shampoo if you were planning on taking a shower.
On a Viewliner the upper berth has windows and is roomier than a Superliner top bunk. Therefore, when traveling on a Viewliner, I always ask the attendant to make up the top bunk for me to sleep in, as opposed to the bottom bunk. That way the lower berth is still in the daytime seating configuration, which gives me some space before bed to read and use the facilities if need be. Also it's nice to be able to sit upright and use the extra space in the morning while waiting for the attendant to come by and make up the room.
That's all I can think of at the moment. I'm sure others will have more to add. Oh yeah... don't forget to tip your attendant for decent service! Enjoy!