Depends on the company and airframe. Of the ones I've been on:

FedEx 757: Two cockpit jumpseats, one bench for two closer to the door. The bench is REALLY tight
FedEx MD11: 2 cockpit jumpseats, 2-4 Business class seats in the back. Padded cots for the floor for crew rest.
FedEx 777: 2 cockpit jumpseats 4 business class seats, 2 bunks.
FedEx A300: 2 cockpit jumpseats, 4 FA style fold down seats aft.

UPS 767: 1 Cockpit jumpseat, 3 FA style fold down seats on aft cockpit bulkhead, one right next to the lav.
UPS MD11: Same as FedEx
UPS 747: 2 Cockpit jumpseats, 6-12 business class seats on the upper deck (depends on if it's a -F or -BCF), 2 bunks.

Priority for the bunks is usually:
- Operating crew for 3 man crews
- Online JS
- Offline JS.

I've not been on any supplemental 747s for a flight, but I'm 98% sure the configuration will be the same.

As I recall:
-FedEx you can book a guaranteed seat up to 24 hours in advance of departure time for each leg. Call ahead to cancel if need be. DO NOT NO SHOW or you could get your company blacklisted. Casual attire (i.e. jeans) permitted.
-UPS you can book up to ten days in advance but can be bumped by UPS pilots up until boarding. Business casual or better, or uniform.

99% of the crews have been top notch and very friendly. If you want to sit up front, they'll usually let you. Have seen some amazing arrivals into ANC from the middle seat of a 777/MD11/747, etc.

Depending on the length of flight there's usually catering. Don't mistakenly eat the Capt's meal. Don't ask me how I know.

Then there was that one time where the Capt introduced himself as an F-14 pilot with x amount of carrier landings (no joke).

The first time I went through either SDF or MEM the Capt gave me the tour of the Ops center and who to talk to and where the quiet rooms were.

If you can make the logisitics and schedule work, it can be a far better experience than the pax carrier (well, except for that bench seat on the 757. Ooof.)

FDX JS Finder: http://www2.alpa.org/fdx/jumpseat/
UPS JS Finder: http://inside.ipapilot.org/tools/jumpseats/index.php