Layering

Everybody knows you can put glitter/sparkles on top of any colour nail polish. This is the most simple kind of layering. Other types of layering include gradients, glitter gradients, jelly sandwiches, and putting a super translucent colour on top of another colour, let's just call this translucent layering. 

Gradients are one type of layering. Some people refer to a gradient as ombre, but an ombre mani is different than a gradient mani, and I'll be posting on the difference soon. 
Glitter gradients are a combination of glitter/sparkle and gradient layering (obviously). I really like them because of how easy they are. The only annoying thing though is how long they take to dry! A ton of layers = a ton of drying time :( Some glitter polishes have a very thick consistency and take forever to dry. They're so thick because that's how the glitter is suspended. 
Colours used: China Glaze - Pink Voltage (base), Orly - Tiara (glitter) 

The next layering technique I have is what I call translucent layering. In this method, you use an opaque colour as the base, and put a very sheer, translucent colour on top. The top colour will alter the look of itself and the bottom colour, producing an entirely new colour. You can experiment with all different kinds of combinations, and see what you like. Here is a combination that I really love. 
Colours used: unnamed dark blue metallic Claire's polish (base), Sally Hansen - Rock 'n' Roll (translucent glitter) 

Some people may be familiar with jelly sandwiches, but many are not. A jelly sandwich is just a layer of glitter sandwiched between two layers of polish. The top layer is very translucent so you can still half see the glitter through it, and the bottom can be the same translucent colour as well but it doesn't have to be. Other colour combinations would be awesome! The point is for the glitter to look suspended on your nail. They're very pretty and extremely simple. 
Credit goes to Colores de Carol for this picture. I attempted to do one but I didn't use the right glitter (it's old and thick, and takes F.O.R.E.V.E.R. to dry!) Click here to see her post about the nails below. 

Glitter or sparkles on nails is pretty, but it's pretty boring. And since it's so easy, I find it to look very amateur and young. (I don't wanna say childish cause that's not what I mean but as I've gotten older I prefer classier, more sophisticated, more elegant, and somewhat simple manis.) I still like to do glitter very regularly, in fact I just did it not too long ago. Glitter/sparkle layering is best when it's subtle, at least to me. 
Colours used: China Glaze - Turned Up Turquoise (base), Finger Paints - Art You Wondering? (sparkles) 

Have fun layering and don't forget to let each coat dry COMPLETELY before you apply the next coat!