You can always give responses to your responses so far, thanking people for their input, which people always like to get - this is a good way of giving something whilst getting something back.
Otherwise, there's no harm in saying something like 'Thanks for all the responses so far, any feedback on...' and detailing particular areas you're keen to get feedback on - specific questions will much more likely garner responses. If you just ask for feedback, people might think 'well, s/he's already had some, maybe s/he should come back when s/he's acted on it'.
Further to that last bit - acting on the responses so far and posting a new mix is another good way of keeping interest up. If people see you taking on board and trying their advice, they will be much more interested in participating in the dialogue on this and your future postings.
And finally, the best way to get help is to give help. If people see you around the boards giving a bit of feedback and opinions on mixes and any help you can give anywhere else on the forums, you will get lots lots more responses on your posts. For example, if you look at mix postings by some of the regulars here, they quickly get up to 10 pages of responses because they put so much into the forums that people jump to offer them help/responses.
Sorry for the essay - hope this helps.
