I got a Roswell Park handy dandy reusable score card. It rocks.
You mean like this:
http://www.roswelldiscgolf.org/store/index.html 
I don't know how you "in your head" guys do it, and I wonder how accurate you are.
I actually find I can accurately replay a round in my head afterward, at least within a few hours of finishing. Wouldn't claim to be able to do it for two rounds the same day. And with my handy dandy RDGC reusable scorecard, I don't have to
.
Example -- Monday night singles this past Monday at ERP from the blue tees:
Hole 1: tee shot (Stingray) off a tree on the left side of the fairway into the shul on the left. Second shot (Star Roc) didn't hyzer enough and ended up 50 feet short of the log across the fairway, off in the crap on the right. Third shot was a lovely slight anhyzer with a Star Mako, low, to bend around a tree and then skid along the ground to within a couple of feet of the basket -- drop in putt.
Hole 2: Tee shot (Mako) just barely missed getting around and under one of the trees between the tee box and basket, kicked back across the creek and landed dry on the tee side of it. Yanked my upshot (R-Pro Dart) right and ended up in the gully way to the right of the basket, in a position where I could not see even a sliver of metal on the basket, despite only being 30 feet or so away. Bent an anhyzer putter shot around and dropped it right under the basket for a second drop-in four.
Hole 3: Champ Leopard tee shot that I screwed up badly somehow -- ended up precariously balanced in some dormant kudzu vines near the top of the mulch pile on the left side of the fairway (between 3 and 5) -- not a terrible position distance-wise or in terms of line to the basket, but with no way to get a solid foothold. Mako approach shot way short and to the right as a result of having no footing. Putter shot from 80-90 feet out hit the top band and rolled a short way, leaving me with a 15 ft putt for yet another 4.
Hole 4: Woods to right right off the tee, just past the break of the slope. Tomahawk second shot just barely got clear of the crap. Hyzer shot with the Buzz was short, bounced off the mulch pile at the top of the slope down to the basket and rolled all the way down past the basket into the wasteland between the basket and the fairway for 6. Putt skipped off the top rim of the basket and landed 5 feet away. Drop in.
Hole 5: Stingray tee shot on line but short. Putt rattled chains but didn't stick. Comeback putt from about 18 ft went home.
Hole 6: Star Skeeter 3/4 way up the fairway. Putter upshot w/in 3 ft of pin. Drop in.
Hole 7. Yanked Star Stingray right, almost clearing all the trees but catching one late that bounced it across the fairway and halfway down the hill in the shul on the left. Tomahawk with Champ Leopard almost, but not quite, into the clear. Hit a tree with the Mako, leaving me still 60 ft out. Putt was perfect flight path, just about the width of the disk too much to the right. 15 ft comeback putt.
Hole 8: Mako that snaked through some trees in the shul on the right side of the fairway, down to my standard landing zone area at the turn of the dogleg. Beautiful Mako upshot to about 12 ft out. Chanked the putt off the top band. Slammed putter into chains so hard it bounced out. Drop in for a five that should have been a three.
Hole 9: Tried my anhyzer line through the narrow gap on the right but missed the line and hit a tree halfway down. Mako upshot aiming for the same gap hit another tree 40 feet farther on. Yanked my R-Pro Dart right from there, into the trees across from the base of the large felled tree behind the basket. Putt off the top band. Drop in for another 5.
And so forth (we actually started on 10, so that was actually the second nine). I don't know really do anything special to fix the details in my mind - just have a facility for remembering stuff that's of no real importance.
