An expression or saying meaning "don't celebrate too early"

Don't sell the skin before you've caught the bear.

Never sell the bear's skin before one has killed the beast.

The die hasn't been cast (yet).

The final/last word hasn't been said/spoken.

It hasn't all been said yet.

It ain't (all) over (and done) yet.

One must not be too hasty in one's rejoicing.

I think this is a useful saying that may fit what you are looking for

Don't count your chickens (before they're hatched)

something that you say in order to warn someone to wait until a good thing they are expecting has really happened before they make any plans about it: You might be able to get a loan from the bank, but don't count your chickens.

"It isn't over till the fat lady sings".

Also, "It isn't over till it's over". (Politicians and journalists in Britain and America alike trot out this platitude all the time when elections are on.)

"Don't speak too soon" - could also work here.

speak too soon

Assume something prematurely, as in I guess I spoke too soon about moving to Boston; I didn't get the job after all .

And then there's the Yogi-ism, It ain't over till it's over.

Yogi-isms are the sayings of Yogi Berra,a famous baseball player and manager. http://en.wikipedia.org/wiki/Yogi_Berra