1- The clothes worn in salaah must be taahir (cleen).
2- The body of the musallee must be taahir.
3- The place of prayer must be taahir.
4- It should be the correct time for salaah.
5- The musallee must be face towards Qiblah.
6- Niyyah for salaah should be made.
7- The awrah must be properly covered.
source : Fiqhul Islam,Deeniyaat Book 3