First things first: go to the init file and turn on the worldgen rejection log option to YES. This will make a log that explains why things are being rejected, and can help you (or someone on this forum) make educated guesses on how to fix it.
The other important thing is to go through the worldgen parameters and turn off all of the "minimum tiles/regions" parameters, which are criterion by which the game rejects maps... you will want to leave the minimum number of rivers to a decent amount, though (I don't think it can cause rejections).
Other things that can cause rejects are not having enough places for civs to seed in. This can be caused by not having enough of a particular biome (not enough mountains for dwarves will cause a reject unless you set the params so you don't need a fortress mode controllable entity) or by having the evil factor turned up so high that civs do not have enough available regions to live in.
For oceans, put massive weight on the lower 25% of the altitude range, and make the mesh low (I guess). Tweak this along with the x and y variance for altitude until you get oceans and land masses of the size and shape you want.
Same goes for volcanism more or less. You can also set the min number of volcanoes high (say 200 or so for a large map- careful though, because this can cause rejections if the number of squares with 100 volcanism is not high enough). This makes 100 (max) volcanism squares turn into volcanoes until the minimum number is met, once the erosion cycle is over (I think).