Perfekt is used mostly in spoken German and is formed with an appropriately conjugated helping verb, sein or haben.

Präteritum is mostly written, but also heard in conversation.

Words like wissen, sein,and maybe denken have I heard.

- wusste, war, dachte - 

Warst du schon einmal in Deutschland?  instead of Bist du schon einmal in Deutschland gewesen.

I imagine this can vary? 

http://www.nthuleen.com/teach/grammar.html

This site has some useful worksheets.