I get asked this question a lot, and with the economy the way it is, oil prices rising and thus fares going up, people want as much value as they can when they travel. For most people, this means the best legroom for as little fare as possible. I'll add the disclaimer here that various strategies work best for different airlines, however a combination of these will probably give you the best chance..
There are several ways to increase your chances of scoring that coveted extra-legroom bulkhead seat.
Some airlines have the frequent flyer programs where you can have a 'preferred seat' put into your booking profile. The only problem is, these seats are popular and if a higher-tier member also requests these, you will miss out.
Web check-in is increasingly popular, so this may be another option, but you have to be quick!! Bear in mind depending on the aircraft configuration, some of these seats may be classified as 'emergency exit row' seats and depending on the airline may not be selectable during online check-in. (Seat Guru may be able to help you check this & have great reviews on leg-room and other in-flight amenities.)
If all else fails, try to check in as early as possible, and ask politely whether any of those seats are still available. As they are popular you may be disappointed, ask only once! You would be surprised how far being pleasant to check-in staff will get you. Always answer questions about exit rows honestly- people sometimes 'hide' reasons they think will prevent them having the seat they want- but even if you get it, the crew responsible for that row can still move you out if they think you shouldn't be sitting there.
These days, it's become increasingly common for airlines to charge a premium for bulkhead seats as they are so popular. In a worst case scenario, you may just have to pony up the cash to get that seat. Sold out? It's supply & demand baby! I often hear complaints from people who are tall or otherwise feel they need the legroom more than others. All I can say to that is, the onyl fair way for an airline to allocate bulkhead seats (aside from frequent flyer loyalty) is by a pay-to-use system. You want it, you pay, and as with many other things in life, it's first in, best dressed.
One last thing- bulkhead seats (especially on widebody flights are often also the location of baby basinettes; if you still get one after all the parents have been allocated, think to yourself do you really want to be sitting near the crying babies all flight!!! :)