I'm using a Brooks Swift on a Nitto Crystal Fellow seatpost, and I'm having ongoing problems with the clamp of the seatpost gradually slipping backwards on the post, lifting the nose of the saddle upwards as it goes. (The rails are secure in the clamp, it's just the clamp that's moving.) I have the saddle all the way back, which I'm sure makes it worse, but I'm not all that heavy of a rider (170#), FWIW.

Any suggestions as to how to avoid this? Roughen up the mating surfaces with a bit of sandpaper? Get a longer Allen wrench and torque the bolt more? Something else?

I had the same problem with titanium seat rails and carbon seat clamp on the post. I used loctite blue on the rail and clamp.....it worked.

It isn't the rails moving in the clamp, it's the clamp itself moving.

I used the loctite on the rails, clamp, screw threads that torque down the clamp and the notches that position the tilt - fore/aft position to keep mine from moving. This was a little drastic and it needs to be in the correct position because it did not move after I did all of that. This was the only thing that worked for me.....and I was tired of messing with it so that was my last resort.

The Crystal Fellows is a single bolt and non slotted clamp If I remember correctly. I have the same problem with Swallow all the way back on a Kalloy knock off. I think without the grooves/slots we are fighting a loosing battle. The only thing i can think of trying, but haven't yet, is some carbon/friction assembly paste. If that doesn't work I am going for a Velo Orange Grand Cru. It looks elegant, has two bolts and a whopping 30mm set back.

Yep.

FWIW I find a Plain type seat post and the Brompton Penta clip works perfectly..
[Wall Bike even cherry picks them to sell, thru their store, though they dont sell the Bikes ]

I use the old 2 bolt Campag seat post with my Brooks Pro. Campag's wrench [771] for the bolt heads on top
has an open end wrench that fits the Brooks tension nut.
Tullio, one would presume, designed the seat post for Brooks saddles ..

Appears that the feeling was mutual. The standard Brooks tension wrench also fits the locking nuts on old Record brakes.