People have suggested beam racks, but they're a real pain because they swing from side to side behind you. Especially if you like going fast and taking corners and turns.

The solution for the op is to buy a rack that's designed to go onto a bike with no rack mounts.

On the cheaper end, the Axiom Streamliner disc:
Amazon.com : Axiom DLX Streamliner Disc Cycle Rack, Black : Bike Racks : Sports & Outdoors


On the more expensive but super lightweight end of things, the Bontrager BackRack Lightweight:
Bontrager: BackRack Lightweight (Model #08214)

Here is a picture of it on a Trek Domane, a full carbon bike with no rack mounts:


There are other options as well, but if I try to list them all out my post gets really long.

Basically they work by attaching to the bike in 2 places - on the bottom through the wheel skewer, which is designed to hold all your weight so it's a solid point on the bike. And at the top under the brake mount, a point designed to withstand the forces of you hitting the brakes hard as you're going down a hill, so again another solidly designed point on the bike.

I would add one more thing. If what you carry back and forth is relatively lightweight this works great. Like I just carry an extra layer (warmer in the evening heading home than it is in the morning), maybe a rain jacket sometimes if it's iffy, lunch. That kind of thing. These works excellent for something like that.

But if you're trying to carry something fairly heavy - like a heavy laptop - you probably don't want to do this with a fast twitchy race bike. If you put significant weight on the back it makes the handling real weird and possibly dangerous. I own a high end full carbon race bike - carrying like a rain coat is no problem, but carrying something significantly heavy in a pannier is a real problem for the handling of the bike. If you need to carry something heavy better to get a more stable handling bike.