There are a couple of things you can do. All of which I've done!

--I order things I know I use all the time early in the season in bulk. I don't loose much and by the end of the season I have very little left. I order them from a wholesaler two states. My yard looks very lush for about 2 months.
--Send out feelers to other designers in the same situation--order together. Just make sure you get payment issues out of the way up front.
--Justify greater mark-up for your plants due to the following reasons: you took your time to hand pick each plant, you went to the nursery to pick it up, you provided the delivery. You should still be able to double your price for the service aspect of providing the plant. When I've had a client get churly over that type of pricing I've explained it to them and they have no problem with it--in fact they thank me for the service.
--Send in orders and hire a driver for the day at an hourly rate that will be much less expensive than yours! Even if you rent a cargo van, it will be less expensive.

The nurseries here will gang orders together on a truck coming to the same area.

I converted my work trucks to burn on waste vegetable oil.

at this time I'm getting roughly 90mpg on diesel fuel (in an 8,800lb truck).
I'd get better milage but most runs are local

I have done a couple of the things MRR suggests -- stockpile the plants I use most, and make sure the clients understand markups are for having their designer hand pick and deliver plants for their job. I've never pooled an order with other designers, but that might be a viable option if you are in communication with colleagues and fellow professionals in your area. I may pursue that too.

Thanks, MRR.