I generally carry way too much stuff around to games. I'm trying to minimize it all, and here are some ways I've found or will be trying.
If you are running a system that doesn't focus on battlemat-oriented tactical play, or the evening's adventure doesn't require that style of play, you don't probably don't have much to carry to the nights game: maybe just a single rulebook, your notes, and your dice.
If you are using a battlemat and miniatures, you can cut down on what you have to carry around.
Just the core rulebook.
Just needing to carry one small rulebook makes things much easier. Savage Worlds: Explorer's Editon and Big Eyes, Small Mouth score high here, as does Rules Cyclopedia D&D and, given the pamphlet size and low page count of the three core books, Original D&D.
Use flat paper figures and separate bases.
I've never been one for painting miniatures, unfortunately, but I have found that part of the fun of many roleplaying games is moving miniatures around on a battlemat. (The kids like them too.)
Now that color printers that print on cardstock are cheap, paper miniatures are practical and good looking. But how do you transport them? It's great that they're way lighter than metal or plastic miniatures, but if you actually cut out and glue up the common triangle-from-the-top and triangle-from-the-side they still take up a good bit of space, too much to take on a business trip, for instance, and they're easily crushed. The “T”-from-the-side paper miniatures can sometimes be folded at the crossbar of the “T”, but they tend to get bent when carried together. If you don't glue the paper minatures you can carry them flat, but using paper clips to hold them in their triangular or “T” shapes it is just too fiddly and time consuming.
So, what I recently started doing was cutting out and gluing together just the front and backs. This gives me flat, stiff standups that can be easily stored in an envelope, but that together with separate plastic bases provide nice good looking 3d miniatures. The plastic bases themselves are sturdy and can be transported in a small bag; even a hundred of them can fit in my computer bag without taking much space. And the flat standups in an envelope travel easily without being crushed or bent.
At some point I should try counters, like Fiery Dragon's Counter Collections, which have the complete D&D 3.5E SRD monsters covered.
Put all needed opponents/monsters on index cards.
I recently got a printer that can print directly on cardstock and 3×5" cards, and I love being able to carry around the opponents and deal out just the ones I need for the current encounter. It greatly reduces papershuffling in the middle of encounters, too, and cuts down on the amount of junk on the table between you and the players. And you can carry them on in a shirt pocket on a pinch.
Don't forget stock opponents.
Having a collection of stock opponents to fall back on really helps. Sometimes just shuffling the 3×5" cards gives me ideas.
Flip Mats and dry-erase markers for battlemats
I got two of Steel Sqwire's original Flip-Mat™ battlemats. They've got 1 inch squares on one side and 1 inch hexes on the other side, and they fold up flat to 8×10 inches and unfolded are 24×30 inches. Two of these fit in my computer bag without bulking it up noticably, and work wonderfully with dry-erase pens. (No more stains from wet-erase remains!) The only problems is that since they fold, they have creases, and don't lay pefectly flat. However, I've used these with paper miniatures and unless someone slaps something down in the middle of mat or really jars the table things are pretty stable and the miniatures don't fall down.
Printed Cardstock Tile Terrain instead of battlemats
I recently bought a bunch of PDF printable terrain tiles from Skeleton Key Games. They make nice looking terrain and they are all standardized shapes that I can print on cardstock on an inexpensive inkjet printer and cut out. Since they're all the same size and shape they stack flat and since they're cardstock you can easily carry them in a standard 6½x9½ manilla envelope, and two or three envelopes easily carry enough for an evening's play. The disadvantages are that tiles sometimes curve, which tends to lead to the tiles moving around more as they're laid and played on.
As a side node, the various tile sets published by Wizards of the Coast for use with D&D, Star Wars, and the associated miniatures battle games have a great graphical appears and are very heavy, sturdy solid cardboard (not corragated), so they make a great battlemat. Unfortunately, they're all different sizes which makes it difficult to pack them compactly, and the thick cardboard which makes them so stable in play also makes them surprisingly bulky. (And I can never figure out easily if I've actually collected up all the pieces after the game.)
2008-08-20 01:29:26: I'm still undecided as to whether these are slower and less convenient than the Flip-Mats. They do look neat, though.
Stones/bennies/chips/status chips, playing cards, etc.