Briefly & clearly describe what you want done. If your mission just has to be complex, consider making it an appeal with no ‘required’ action. Often you will get better results by simply asking.
Offer a small number of rewards; then Rerun to get more time on the first page of the mission list. Being on the first page is probably the biggest factor in getting your mission done. Staying on the first page is a question of how many folks post missions after yours. You have no control over this…perhaps a discussion with the roof gods to help your eaves.
Make sure your mission is consistent with your brand. Don’t be that person who loses his mind about what you can ‘make’ other people do with your mission. Use your mission to reinforce how you want folks to remember you.
If you feel that you must warn the mission thieves, keep it polite and brief & live up to what you say. Don’t threaten to block or provide negative feedback if you don’t plan to check for mission completion.
Never ever trick folks into your mission. If your mission is going to take 5 or 10 minutes; only folks who are bored and lonely are going to do it. If you didn’t warn that it was going to take a long time, get ready to enter CAPTCHA’s & have rotten dividends because we are all going to give you the negative feedback that you deserve. Treat people right and all will be good with your world.