AS for coding. Sweeps have a definite start and end date. You also need to determine the entry type, daily, weekly, or once.
Then you need to decide on the entry method. Do users need to preregister or can they just surf in and enter an email addys and continue. One of the things that will determine this is the actual reason for the sweep. Are you merely harvesting addresses for a email campaign or are you trying to gain a user base. Or are you making a revenue producing ad display engine. Those above will determine how tightly you keep registration and entry type.
Next comes, what exactly are you giving away and what is its value. Value is not what you pay for it but its real world retail worth. Along with this is the geographical area to be allowed entry. This determines the rules and regulation that need to be followed.
Before I do a bunch of useless finger work. If you are US based let me know and I can tell you, as IANAL, what you need to be aware of and the pitfalls to stay away from. If you are not US based I cant be of help in the RR area.
HTH
Teamatomic