grappling in itself is the best way to work all the right muscles. so if its a toss up between working out at a gym and getting more jitz in, go for jitz.
yoga seems to compliment jiu jitsu really well as it tones and strengthens your muscles, increases flexibility and can help with strong joints etc. (vague description sorry, im sure others can add to it)
i think core workouts are super important to everything, general life, jiu jitsu whatever. so not just sit ups for your abs but make sure you work on your obliques, back muscles such as your lumbar (low part of your back) and your abs. there are exercises you can do to work your deep abs which can add that bit extra spine support and core strength.
here is eddie talking about flexibility and squeeze game.
http://www.youtube.com/watch?v=OktcKhtMQ88 its really a good thing to work on holding a strong squeeze for a long time. that kind of skeletal strength can shock people.
even though we mostly all do no gi i think hand strength is really important as well. so what i did is buy two stress balls which i squeeze and hold to improve hand strength, but there are proper rubber rings of different strengths that you can also use.
apart from that, being explosive in your training can be really helpful aswell.
its not detailed exercises sorry but i hope it helps. i can post up more detailed stuff if you want. im sure others will too.