Mixing is a mix of science and art. It takes a lot of practice to be able to mix well, but there are also some principles that can help make the process easier.
Let's talk about the mix first. It is important to have a mix that sounds good before you start adding effects. Make sure your track has dynamics, it breathes and doesn'st sound over compressed or too quiet in some parts. Also make sure there are no frequencies fighting for space in the mix (i.e.: bass fighting with guitars) — this makes for an unbalanced mix where nothing really stands out except when one instrument goes solo; not exactly what we want here…
Now let's talk specifics on how to add effects:
Reverb (not always but often). Reverbs can help glue all together by creating reflections of the sound in a certain space. Usse reverb on instruments that are playing together, especially if they're all in the same frequency range (e.g.: guitars and bass). If you add reverb to each individual instrument it will start to sound messy and muddied.
Delay. Delay can be used to create a sense of spaciousness or depth in the mix as well as make an instrument stand out from the rest.
Another common effect is compression. Compression can help control the dynamics of a mix and also make tracks sound more even overall.
EQ is another important effect to use when mixing. It can be used to shape the tone of a mix and make certain instruments or frequencies stand out more.
Modulation effects such as chorus, flanger and phaser in my mixes. These effects can add depth and interest to a mix.
That's it for now! Hopefully these principles will help you mix your tracks better. As always, experiment and find what works best for you!