This is something a co-worker of mine says all the time.

the 'crystal ball' is what a palm-reader/psychic lady[person] uses to predict the future. So "sorry, I don't have a crystal ball," means that I am unable to see into the future, as a psychic SUPPOSEDLY is able to do.

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Note added at 4 mins (2004-05-22 19:11:32 GMT)
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Or, jokingly, \"sorry, but I left my crystal ball at home today\"

In a more "negative" way of thinking... might be good in some contexts

Do you think you are too young to know this Doris Day's mega-hit song?

Que sera, sera
Whatever will be, will be
The future's not ours to see
Que sera, sera
What will be, will be

Don't take me too serious. Just for your amusement.

;~}

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Note added at 2 hrs 1 min (2004-05-22 21:07:51 GMT)
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IT AIN’T OVER TILL THE FAT LADY SINGS
[Q] From Caroline Francis Carney: “Do you know the origin of the expression It’s not over until the fat lady sings? I believe that it’s a reference to opera. My friend Paul credits this saying to Yogi Berra in reference to a baseball game. Who is right?”
[A] It’s usually taken to be a reference to opera, so many performances of which seem to end with a set-piece aria by a well-built soprano, but its recent links are mainly with sport, so much so that some people think that is the origin.
Commentators do often say the phrase to remind people that it’s the final result that matters, often in a spirit of reassurance to the supporters of the losing team. It has been suggested that it was the brainchild of the San Antonio TV sports editor Dan Cook. It’s probably not one of Yogi Berra’s phrases, though it has the same sense and much of the style of his “It ain’t over till it’s over”, which he is said to have coined in 1973.
These comparatively recent sports’ associations disguise the fact that it is actually a rather older expression, which occurs in several forms: “It ain’t over till the fat lady sings”, “The opera isn’t over until the fat lady sings”, or “Church ain’t out ’till the fat lady sings”.
This last version appears in a 1976 booklet entitled Southern Words and Sayings. Ralph Keyes wrote a book with the title Nice Guys Finish Seventh in which he says that several informants recalled hearing the expression for decades before it suddenly became nationally known in 1978. The use of church here suggests that its origin wasn’t tied to opera, either, but to church-going.

Idiomatic phrase in common use in UK

Another suggestion, of many.

Don't Count Your Chickens Before They Hatch
Don't count your chickens before they hatch. Meaning: You use this
proverb to warn someone not to plan anything that depends on ...
humanities.byu.edu/elc/student/idioms/ proverbs/dont_count_chickens.html

Idioms: don't borrow trouble -- don't give me that line/story etc.
... don't count your chickens before they hatch, do not expect all plans to be successful, wait until you get the final results, "Look at the sales I'm going to make ...
home.t-online.de/home/toni.goeller/idiom_wm/id167.htm