A fundamental for all good DJs is the ability to use compressors to balance out the sound of their mix. A compressor reduces the level of sounds over a defined limit. This is usually done to increase the volume of quiet sounds and decrease the volume of louder sounds. Compressor use is a particularly good technique when performing in crowded, noisy venues, or in front of very large audiences. Many television commercials make extensive use of compression to achieve maximum loudness within the legal limits.
In the context of mashups, compressors take on an increased role, as tracks are usually "mashed-up" (simultaneously mixed) together from different sources. For example, when mixing a loud rock vocal sample with a down-tempo hip-hop beat, a DJ will need to adjust the compression settings of each track to reduce distortion and limit ranges.
Combining elements from different tracks is an adventurous task and the outcome can be something of a crapshoot. Creating a good mashup track is often a process of trial and error, but there are several ways to reduce the amount "error" and shorten the length of the "trial" period.
Make sure to obtain your samples from high-quality sources, such as a direct rip from a CD or a lossless format like FLAC. This will reduce the amount of distortion in your mix and help ensure a similar volume level for all tracks. When selecting source material, be sure to explore unique and original places to find music, such as local record shops or old cassette tapes.
When organizing your instruments and samples, particularly those with vocals, it helps tremendously to sort everything by tempo. Many audio editing programs include features to do this, though it can also be done by placing samples in matching folders. Vocals can be matched with samples of a different tempo, but can also be awkward if the pitch is changed too much. Try to match samples as close as possible to their original tempo, unless a distorted effect is desired. Differing beats can make a good mix sound like it is out of sync.
When working with tracks that are at an extreme tempo difference, like when mixing a 95bpm hip-hop beat with a 170bpm drum and bass track, try to get the slower track to play at half the speed of the faster track. In this way, timing structure will usually hold steady and allow for the freedom to add another component to the mix.