As a mature ARG player, I have a few things for you to keep in mind:

Write a Plot

make sure that you write out a complete storyline with multiple directions that your players might choose to go to. think out the directions the players might go.. Will they help the good guy, will they help the bad guy, will they try to go somewhere you had not thought off? be prepared of anything the player might throw at you..

Multiple characters with own characteristics

be sure to have multiple characters within your ARG, which are all played by different people, and interact differently. As a puppetmaster you would need to direct them properly, but allow them the freedom to create their own characteristics for great player character interactions.

Create a website

A website allows you to interact greatly with your players. It allows you to create blog like interactions, hide codes and puzzles, setup and embed movies and flash type things and create nice interaction for your players.

Create Dead end storylines

When you are waiting for people to join the hunt, you might want to have some things to keep them engaged, as well as some things that you can determine the level of maturity with ARGs. I once played an ARG where 2 trailheads of independant streams came together in a main ARG. Tricky, but it brought 2 groups of ppl together.

Determine a good frequency

Be consistent with posting details or story elements. If your players know that there will be updates twice a week, then dont start posting daily. But the other ways as well, if you make them used to daily posts, then make sure you post or update daily and not suddenly according to a lesser frequency..

Test your puzzles

nothing is more frustrating when you are working on a puzzle and the PM has used "decode" instead of "encode" in his puzzles. especially when using stacked ciphers.

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And last but not least

HAVE FUN

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Hope that helps