How to Make a Proposal

Know where to submit it (either direct to the list, or to a Working Group first)
If your proposal overlaps with the responsibilities of a working group, send it to the group before posting to the list. They may be able to take action on it themselves; otherwise they'll ask you to present it to the General Membership.
If the proposal doesn't fit with any particular working group, or if a you feel the WG hasn't given your idea a fair hearing, you can present it to the General Membership yourself by posting it on the list.
Write a clear and thorough proposal.
Provide all the relevant info in your writeup (rationale, details, cost/revenue projections, help needed, timeline, etc).
Make sure each option that needs to be voted on is presented clearly.
If you want the co-op to authorize spending, include a budget.
If you have not begun to do research, your proposal should include questions you'd like answered before you can begin.
Post and present it.
Send it to the discussion list, (not Alerts), and
Post a printed copy in the store, both by the deadline (store closing, 8 days prior to meeting)
Bring 6 copies to the meeting for people to read, and be ready to answer questions. If copies are not brought, the proposal will not come to a vote.
Proposal Template: Proposal Template, text file.

Tips for an Awesome Proposal

Consider the total effect of your proposal. The devil is in the details. Keep it as concise as is possible.
If your proposal is not a straight up/down vote, but contains options, submit your votes prior to the meeting. Send rough copies, or even just a summary of your idea to working groups, admin, the discussion list, or long-term members. Feedback and discussion prior to voting makes for more efficient meetings, and better proposals.

An archive of past proposals is kept in the members section.