The five condtions for a hadith to be sahih
1. Complete isnaad (a continues isnaad all the way back to the Messenger (Saw) or a companion from begining to end)
2. Narrators have to be Trustworty,
3. Dabt:They have to have a Strong memory
4. Shadahdh: meaning it cannot contradict a more reliable hadith that is reported a larger number of narrators or even by one transmitter of higher authority.
5. Ilal: Free from hidden or obvious defects.
for more information on the basics of sciences of hadith, refer to
http://islamicbookstore-com.stores.yahoo.net/b2872.html (intro to sciences of hadith by suhayb hasan) – he also has a cd set explaining it: http://www.ilmquest.org/pc-142-106-sciences-of-hadith-8-cd-set.aspx
this book my mustafa azami on studies in hadith methodology is also a geat intro to the topic:
http://islamicbookstore-com.stores.yahoo.net/b2913.html