The internet is an absolutely lovely device, and can do a lot of the prep work for you if you know where to go. There are sites that will randomly generate dungeons for you - complete with placing traps and encounters. There are sites that will do random treasure generation for you. There are dozens of sites that will give you random lists of names for anything and everything under the real world, fantasy world, and/or sci-fi world sun(s).
The more I GM outside of L5R the more I find myself using these tools, and they work well. One thing in particular I've had a lot of use for lately is a random personality generator. Having a personality for NPCs that could feature prominently in your game is a good way to give them depth, and a random generator can do wonders to cut out creator tendencies that can make all NPCs of a certain type/role/purpose feel similar.
At present I've been using this one, set to detailed. The detailed view gives a good break down of the qualities I find I need most when running someone. The inner psychology, the way the character interacts socially, how they live their life, what they're currently feeling, how good their self control is, and how honest and honorable they can be. It works well.
Now, to be clear, I don't just take the first one I get and go with it. I compare the psychology to what I already know about the character. If an NPC is a high level wizard that is going to be the big bad, having a personality that makes them all sunshine and rainbows, while also illiterate isn't going to help anyone.
Instead, I'll cycle through a few and look for a good one. I'll take that good one as a base and slip in some changes based on other things I've seen that I think would be interesting to explore with the NPC in question.
In general I've found the notes quick and easy enough to hold onto for NPCs, and it really does help me give characters more depth. I particularly like when you get traits that conflict with themselves - i.e. hates being alone. Needs alone time regularly. - Or when you get ones that just add fun to things. Such as a manipulative, emotionless villain who is trusting and prone to putting things off last minute.
Give it a whirl. See if you like it. See if it is for you. There are other ones out there (just google Random Personality Generator and you'll get a bunch), but I like that one I shared a lot.