1. Exfoliate - Use a body buffing brush or glove and an exfoliator with granules to scrub away the dead skin cells.
2. Make sure your skin is well moisturised, but also that the moisture has been fully absorbed by the skin before you apply your tan. Don't use oils as they sit on the skin for too long.
3. The reason you get streaks is either due to lack of exfoliation, or that you have not applied enough tanning product. My favourite is St tropez bronzing mousse.I pump out about four or five pumps for just one calf. http://www.feelunique.com/p/St-Tropez-Self-Tan-Bronzing-Mousse-120ml
4. If you are using a tanning product on your face, a spray works best as it avoids streaks and finger marks. For darker skin I love ModelCo spray tan. I use it to top up my tan at the end of the summer.
http://www.spacenk.co.uk/product/shop+by+brand/model+co/200006962+tan+airbrush+in+a+can+jet+set.do
5. On shoots when models need to be instantly tanned I like to use Laura Mercier bronzing gel for the face with thier cult bronzer Laguna. http://www.spacenk.co.uk/product/shop+by+brand/laura+mercier/200006960+body+bronzing+makeup.
For the body Rimmels sun shimmer is also great for an instant glow.
6. Be careful not to over do it!!!! ( a very commom mistake!!!) On the face it's best to have a flawless application of foundation with a touch of bronzer; unless of course you're auditioning for the next series of The only way is Essex!!!!