I would offer that a lot of it depends upon your client base. As you've probably discovered, there's not a "magic structure", where one type fits all. Here are some thoughts:
A volume discount with a minimum.
You offer them a rate of 90% of normal if they agree to a minimum of 10 hours a month (for whatever contract period), 85% for a 15 hour/month commitment, and 75% of normal rate for a 25 hour commitment. Substitute your comfortable rate and hourly minimums as appropriate. If they don't meet the minimum you still bill them the minimum rate. Typically you'd have them sign up for a term of several months at a time - a bigger discount if they sign up for a longer term.
A tiered structure.
Similar to the above, but instead of making them sign up for a set commitment per month, you give them some level of volume discount for more hours. So, 5-15 hours = 5% discount, 15-20 hours = 10% discount, 20-30 hours = 15% discount.
In either case, though, you'll want to give them a contract with some sort of a service level agreement ("I will respond to you within x hours, and will be on-site within y hours.", or something like that) so they feel comfortable that they're high on your list. I'm not an attorney (consult with one before you do a real contract), but you can gain a lot of client confidence if you're willing to put your name on the line and sign up for something. Just make sure YOU'RE comfortable you can meet those timelines.