1) Make sure you can get your stuff into the bags you are using easily. The Sealine dry packs we used for sleeping gear did a great job and compared to panniers were very cheap. However it was a fag to get the mats and bags inside the rolls each morning (and hang them off the triathalon bars).
2) Make sure you can get your gear and bags on and off your bike easily. This is important at the end of a long day. Being able to get to your tent quickly, can make the difference between getting soaked vs weathering the storm in the dry. Fiddly systems to lock/unlock your bags to the rack can be very frustrating at the end of the day (we now have Ortlieb bags).
3)Pack 'logically'. For example the first thing you will want to get to on arriving at a campsite is the tent. Then the cooking gear. So don't bury the important stuff at the bottom of a pannier.
4) Pack consistently. We allocate one bag for cooking stuff, another for food, one for emergency tools, extra water and high energy food. By the end of the trip we could get to stuff in a hurry without too many problems.
5) If you can afford it go for waterproof panniers. They are brilliant. We managed in 2005 by putting clothes in waterproof bags and bin liners, however the Ortlieb bags are just so much better. We had some torrential rain in 2006 and nothing got wet. I'm still inclined to say spend your money on good panniers even if this means getting a cheaper bike, as usual YMMV.
6) A 4kg tent is not too heavy. Having a separate inner tent that can be put up later is useful in wet weather. It also helps keep the inner tent dry.
7) Tyres with a mountain bike style profile seem to help on loose gravel sections and off road sections.
8) A warm sleeping bag is essential. We had good weather but it often cooled off a lot in the evening and night. Damae was just too cold whereas I was OK, in the same type of bag. We now both have bags we can use in the winter combined with sheet sleeping bags.
9) Don't get your inner tent wet. If your inner tent stays dry then your sleeping bags and mats will also stay dry. If everything else is wet then at least you can get a good night's sleep.
10) Using two litre water bags attached to the handle bars seems to work well. It is also easier to drink on the move than when using a bidon.