I've done a 2500 mile tour pretty much every year for the past 5 years. This is what I've learnt:
Buy a tomtom rider off ebay. Navigating off a damp map in a tank bag is a total nightmare. As are french motorway tolls on a bike (stop-gloves off-pay the monkey-stash change-gloves on-ride away-repeat every 100 miles. A credit card to get yourself a hotel when it pi$$es down is essential: the accor hotels iphone app is really useful for that
Irrespective of the forecast, take wet gear.
French Route Nationales are good fun & not heavily policed, though you will end up seeing a lot of lorries avoiding the autoroutes. Spanish roads are generally immaculate though you may end up playing the "we built that, we own that, we paid for that game on the motorways that lead to nowhere (Pamplona to half way to Barcelona is a prime example)
Plan your route as best you can. A lot of people are using an app called tyre (http://www.tyretotravel.com). Here you can fine-tune your planned route & get a good estimate of how long it's gonna take. We've found that tomtoms in europe are far more accurate at estimating ETA than they are in the UK. Microsoft autoroute isn't.
Once you've planned your route, don't stick to it come hell or high water - go with the flow and be prepared to chop and change based on conditions, how the bikes behave and how you're progressing: even "fixed" ferry tickets can be changed pretty easily.
Try and take a list of your bike brand's dealers with you. Parts are a nightmare to source abroad but a franchised dealer might just take something of a display model to help you out - I've had this with my KTM before.
With that in mind don't use disc locks as there's nothing worse than setting off slightly hung over & feeling the clunk as your lock destroys your disc / caliper when your 800 miles from home in the middle of Germany on a Saturday afternoon.
Alpkit sell some great, cheap, dry bags to line panniers and tail packs with. Their tougher airlock extra bags are also fine for strapping down outside of a pannier for things like tents, sleeping bags and thermarests.
If you must use a rucksack, make it a small one like a camelback - let the bike carry the heavy stuff. Drink lots while riding in the heat.
Earplugs are the difference between being able to cruise at 60 all day or 80. They also make it much less tiring. Buy in bulk from Arco & take enough for two sets each day.
Learn how to use ratchet straps before you have to strap the bike down to the ferry deck. We now use one per side on each bike (the ship'll have them) as we're fed up of single ones cutting into the saddle.
You HAVE to carry two breathalysers each in france, and glasses (with spares!) if you need them. They were looking to force bikers to wear hi-viz. I'm not sure what became of this.
If you or your mates have a set of loud cans put the baffles back in. 2000 miles of WAAAAAAAAAA-WAAAAAAAA-WAAAAAAAAAA or THUDTHUDTHUDTHUDTHUD is terrible for the person behind.
Long stretches of motorway are boring and really tiring. To the point that I nearly fell asleep on the A1 while riding once. Try to do no more than 120 miles / 2 hours between breaks.
British credit cards still fail to work in French petrol stations for some reason so be prepared to take cash.
You riding jacket's pockets are not waterproof, so put your phone and wallet in a freezer bag or proper aquapack.
I have gpx files (or tomtom ITNs) for that part of the world taking in some amazing twisties if you're interested.
Wherever you go - remember it's not a route march it's a holiday.