Find a bike that is comfortable for the long haul; this means a bike that fits and a touring-quality saddle (notice I don't call it a seat). It might feel really great sitting on a blob of Jell-O for a few miles, but trust me, if you start clocking 15 or 20 miles, all of that movement is going to cause some chafing. I find leather saddles the best.
So, pick your iron horse and ride it until you increase strength and work out the things that are going to hurt you after a few miles. It's a bit of a luxury to bring your own bike, so anything you can do to build up your butt and hand tolerance will help, even when renting a decent bike.
If you have found your perfect saddle, consider bringing it to install on your rental. Cycling shorts are a plus because they are designed with seams in places that help prevent chafing. Almost all of them feature some kind of internal padding, known as a chamois, which works well on higher-quality shorts.
Pick a bike-friendly destination. Would you rather pedal around a place like the concrete jungle of Houston or in bike-friendly Portland, Ore.? Be realistic about your abilities. You don't want to find out your weakness while on a beer-and-bike vacation, but that doesn't mean you shouldn't challenge yourself.
Bike-friendly destinations are exponentially safer and more fun to ride. Belgium may be the place that strikes the best balance between being unbelievably bike friendly while also being the beer equivalent of Disneyland. Germany is great as well, but you will encounter some hills.
Set realistic itineraries and goals. The first thing to do is decide if you are riding from point to point or returning to a base hotel. So far, all of my touring has been the return-to-base type; this lightens the bike luggage load. You may be able to find a luggage forwarding service to deliver your bags to the next destination if you really need to do point-to-point touring and don't want to be loaded down with panniers (bike-mounted bags) or a bike trailer.
Try to keep return-to-base trips more interesting by finding loops or different return routes so you don't meet the same cow twice. Make sure you know your limits. Factor in such things as wind and hills, which can make a 10-mile ride seem like 30.
On my Belgian trip, we returned to our base by train on our first day after a very long ride. Be sure to check the availability of rail transport when and where you will need it. Check the fare structure for additional bicycle carriage fees.
Contact the tourism bureau where you plan to ride because they may be able to direct you to outfitters that offer fully preplanned bike tours complete with baggage-forwarding services.
Use a bike/hiking GPS or cycling maps. My Garmin Dakota Hike and Bike GPS worked great in Germany using free GPS maps from www.velomaps.org. The free Velomap for Belgium was not so good; fortunately Belgium has a very well-marked bike path network and paper maps are available at almost any bookstore.
For general information on cycling vacations, check into these resources:
See Rich Ireland's Beers to You blog at thegazz.com.