Acoustic guitar strings are the most important part of the instrument. Over the lifetime of the guitar, they are probably the item that most guitar owners will find they spend the most amount of money on (eventually, the accumulated purchases will probably add up to more than the cost of the guitar itself). There is a lot to consider when it comes to the right time and the right type of guitar strings to purchase for you and your instrument, so here are some things to consider.
Frequency of Change
When was the last time you had your acoustic guitar strings changed? Strings definitely have a peak time. Often, guitar players will find that their strings are dropping out of tune for the first little bit after they are first changed. This is because the strings need to stretch into shape a little bit before they begin to retain the perfect tone, and it does not mean that the strings or the guitar are defective.
If the strings have been on the guitar for a while, though, and they are constantly needing to be re-tuned (for those with steel strings) it might be time to purchase a new set. You should always change all of your strings at the same time, so that they are about at the same age and quality of sound. Before you change them, though, remember that temperature can also effect the ability of the strings to stay in tune, so if it has been cold then you might not need new strings quite yet; remember to store your acoustic guitar somewhere warmer though.
Type of Guitar
The type of guitar, of course, is critical in deciding what type of strings you need to purchase. Nylon strings are much easier to break than steel strings, and they also give out a much less aggressive tone. 12 string players will find that they need to buy twice the amount of acoustic guitar strings as their six string counterparts, and many will also purchase specialized strings in different octaves.
Changing the Strings
I prefer D’Addario medium gauge strings on my Washburn, for a couple of reasons. First, the medium strings are more durable than lights and they also give off a meatier sound without destroying my fingers. Second, and most importantly, they have nice color coded circles that tell me which string I am looking at in the case that the string slips out of the case. You can also tell by the diameter of the string, but only players who are very familiar with their instrument can hope to accurately gauge the diameter and match it to the correct string.
The most useful device for changing acoustic guitar strings is one which will allow you to easily pop out the endpins. There are cheap little devices available that wedge right under the end pins and pop them out, without harming the finish on the guitar. These devices also have turn pins that make winding the strings up on the machine heads a lot easier.
Of course, there is only so much that you can do to make sure that your acoustic guitar strings broadcast the sound from your guitar throughout the area in which you are playing. The heaviest strings will not allow your music to be heard over other instruments in a band; for that, you need the right combination of amps and acoustic guitar pickups.