I would say that if you want investors, you should pay them out at their investment rate (ie, if someone purchase 25% of a your hashrate then you pay them 25% of the bitcoin received minus a small fee and electricity rates.) every so often (day/week/month) based on what they choose.

This, a) means that those investing start getting rewarded as soon as they invest and b) will prompt more investment as their payouts are often, they will more likely reinvest their earnings.

So say I have an ASIC, I would put say 100 shares that are purchasable.  Its hashrate is X.  An investor purchases 10 shares.  Their bitcoin amount rate equals X/10*.95 (5% fee)-(electrical costs)/10.  This means that investors don't have to take a huge risk by purchasing small share amounts, the user running the ASIC recieves a 5% fee for maintaining/etc and the initial cost (if there are enough investors) is 0 or close too.