Because most corporate prediction markets use fake money as their currency, an administrator must think of other incentives besides the ability to make real money. Of course even with fantasy currency, the better you trade, the more currency you accumulate, so it's natural when thinking about how to incent people to participate to offer a reward to the "top trader" in a marketplace.
Unfortunately while sounding good on the surface, this incentive often falls flat as people in positions 5-x feel like they don't have much of a chance of catching up. Unless you have other incentives in place (and you're not creating new markets on a regular basis as Nate blogged about a few weeks ago,) your participation may slowly dwindle.
One incentive that doesn't get much attention is granting "exclusive access" to traders. The most successful format we've seen of this is picking those who are regularly participating in the marketplace, i.e. "20 random traders with 20 or more trades" and inviting them to a roundtable with a (very) senior executive on a monthly or bi-monthly basis. At this roundtable, discuss company issues related to questions being asked in the marketplace. Keep it casual and off the record. Have the executive stress that in addition to carrying out daily duties, participation in the marketplace is another valuable way to contribute to the success of the company. In other words, make people feel...like insiders.
This also applies more generally to internal social networks which often have some sort of point accumulation mechanism as an overt incentive to complete a profile, participate in discussions, answer questions, etc. iPods and gift certificates are nice, but think about saving your money. Especially in a large corporation where so much is dependent on your level and title, feeling like a player for an afternoon may be all the reward you need to offer.