It doesn't bug me if my whole head ends up soaked with water.You might try tying a shower cap or plastic bag around your bun.

but do you really feel like this rinses well the shampoo. personally it's a reason i never tried it. first of all i don't really use shampoo anymore, and seems pretty annoying with conditioner. and then there's the spot where you make the bun that stays greasy.
i'll never become a great scalp washer 

I find doing two braids loose under the ears, then putting the shampoo on the trouble spots without extra water, then lifting the braids up with one hand while dousing the scalp from above with the shower head works the best.  No greasy spots, full rinse acess, and for some reason you don't get as much end soakage as you do with a single bun.  Just takes a little co-ordination.

You've probably heard this a million times already and I even see a thread on it below,  ...but CO'ing can really help with that problem. My hair is in pretty nice condition now but it used to be very, very dry on the ends (from chemicals). Yet, when I went a day without shampooing, it was INCREDIBLY greasy on the scalp. I would have an oil slick on the top and dryness on the ends.

I found with CO'ing, after my hair got used to it, it was more uniformly soft and non-greasy from the top to the tips. I know it sounds counterproductive but when you shampoo, your scalp responds by producing even more oil. It feels stripped and dry so it oils up like crazy. If you can bear the first few weeks (wearing hair up), your scalp may normalize if you try CO'ing. Just my 2 cents!