My NaNo Prep strategy is to pretend I'm not going to do it until the last possible minute, then make a snap decision that I'm going to take a break from whatever I'm working on to do something fun. The problem with this is that I got kind of obsessed with the thing I started last NaNo and I'm still working on it, so to follow my usual strategy I'd probably have to go back to the thing this project is a distraction from. Or, I could follow my more recent 'usual strategy' and just keep attacking this project until I finally get to the end.
As far as planning vs. pantsing I'm definitely a pantser, so my preparation for NaNo is pretty simple. I make a list of characters I know are going to be in it, write a brief description of them, write down some rough guidelines for the setting, then try to write down what the opening image is without actually starting the story. If I know anything else e.g. I might be setting out to find a different way to develop an older idea, I write that down as well. I don't bother trying to develop plot points or anything like that, because at this stage it would be a list of things that definitely aren't going to happen rather than a useful guide.
Also, you need a title. It serves several functions for me - an expression of theme, a thing to name folders and story files, and of course a necessary part of signing up for NaNoWriMo. The 'expression of theme' thing is sort of by accident. I choose titles that sort of fit the story I'm expecting to write, but they end up being somehow integral. For example, one time I used the title Placeholder because I had no idea what I'm going to write. One of the major characters turned out to be the prototype/guinea pig for an experiment the bad guys were running, and another one became a scapegoat of a less-bad guy's embezzlement scheme. Or another time I decided I definitely didn't want to write about a necromancer, so I used the joke title Hold the Necrosauce. You'll never guess what the story ended up being about.
Finally - and this probably sounds either obvious or silly - I don't think you really have to do anything special or different for NaNo. It's more like the distilled essence of the things that already work for you. If you usually plan things thoroughly, you won't suddenly be able to write a fantastic pantsed draft, and if you're usually a pantser it's quite likely you'd end up dumping any planning you attempt. So, I'd say the first step is to figure out what works (and what doesn't work) for you already. Then, figure out how to adapt the 'what works' list to working at NaNo speed and also probably figure out how to avoid falling into the dangerous territory of the 'what doesn't work' list.
- You will get attached to your NaNo story even if you start it as a distraction
- You need a title. Your choice of title will probably affect the story even if it's just a joke or placeholder.
- Make two lists - 'things that work for me' and 'things that don't work for me'. Use the first list to figure out your NaNo strategy.