The Missouri presidential primary will take place on February 5, 2008. After that, the process for selecting Missouri's delegates who will nominate the party's candidate for president in Denver at the Democratic National Convention will unfold at various levels through the beginning of May.
The first step is to be registered to vote by January 9, 2008. The second step is to actually vote in the February 5th presidential primary. If you don't do so, you can't be a delegate. Period.
...Participation in the Missouri delegate selection process is open to all voters who wish to participate as Democrats, who are residents of and registered voters in the political subdivision (ward, township, county, legislative district) which is holding the mass meeting, who are at least eighteen (18) years of age, who declare themselves to be Democrats, and are not members of any other political party. Every person attending a ward, township, legislative district, or county mass meeting shall sign a standardized declaration furnished by the Missouri State Democratic Committee that states he or she meets the qualifications as set forth in this section and indicates the address and telephone number of his or her permanent residence. They shall then receive a pledge of support form to be submitted to their caucus leader at the time caucuses are formed. This form shall also include a declaration of affiliation to the Democratic Party and confirm that they voted in the 2008 Presidential Democratic Primary...
The party will check to see if you voted in the primary - it's a public record. If you didn't vote in the presidential primary and you somehow get selected in the early part of the process you will forfeit your position.