Deciding on when to go may or may not be an option for you. If you are a teacher, for example, you will probably only have two months in the summer to travel and thus, relatively little choice in the matter. But if you do have some flexibility in your life, your decision on when to go should be influenced by where you are going. You will want to look for agreeable weather conditions and most likely avoid peak travel season if at all possible.
Peak travel seasons are accompanied by large crowds and quite often more expensive prices. Their timing varies from place to place. Peak travel seasons usually fall into the most pleasant weather patterns for a particular place -- not too hot, not too cold, little rain -- or when the majority of world travelers have vacation time (probably the greatest factor).
If you are looking for a warmer climate (which allows you to pack lighter by the way), stay away from extreme north and south latitudes in the winter (obviously) and stay closer to the equator. However, you may not want the temperature to be so hot that it saps all your energy. The key word here is moderate, as in moderate temperatures and weather patterns. This is what most travelers are in search of.
Try traveling just before or after the peak season of a region -- often called the shoulder or 'edge' of peak season. This way, you will avoid the huge crowds but still have relatively pleasant weather, cheaper prices and access to still-open attractions.
A well known example of peak season travel is Europe. Europe is extremely busy in the summer (particularly in August when many Europeans take their summer holidays). Traveling here in the summer makes finding an available hostel bed difficult, trains and ferries are packed, prices for almost everything are more expensive, and long lines are the norm at all tourist attractions. However, all hostels and tourist attractions are open, tourist attractions often have extended hours and the weather throughout Northern Europe is generally more pleasant. Europe is best visited in the off season unless you are into any of the above.
If you have to travel during a region's peak travel season, try to get off the beaten path and you will get away from the usual tourist crowds. This might entail walking the back roads of a city, away from the touristy-type areas, or you may want to steer clear of the larger cities all together and experience smaller towns and villages.
You will also find that getting up early and visiting major attractions before the rest of the crowds show up will help reduce your queue time. Tour groups seem to appear out of nowhere on their large busses, causing major congestion as they swarm the hot spots. However, they don't usually like to get up too early and they take more time to get organized, leaving the morning relatively quiet for others. The end of the day is also usually a good time to see the sights, after the tour groups have moved on.
Accommodations are worth more than gold in the busy season. Your best bet is to book beds in advance. Purchase a Hostelling International card before you leave so that you can make hostel reservations from city to city at any HI hostel. If you are staying at a private hostel, try to call ahead and to book a bed. By simply arriving in a city and expecting to find a place to sleep, you are setting yourself up for hassles and headaches.
One travel strategy in Europe is to visit the southern countries like Italy and Greece in the early spring and then travel further north as summer approaches, using the improving weather to your advantage. This will also keep you away from the European tourists, who flock to the Mediterranean in the summers while on their own holidays.