The relativizer (that or a wh- word) which introduces a restrictive relative clause may be omitted in all registers if it designates a constituent other than the subject in the relative clause. In your example, for instance, that/which would designate the direct object of want, so it may be omitted.
The relativizer is usually omitted in speech, except when the speaker is groping for words—in this case, speakers will often include that, and even repeat it as a useful 'filler'. In formal discourse it should be included if your sentence is long or complicated, because including it helps the reader parse your meaning. I myself prefer the appropriate wh- word in such contexts, because that has so many uses besides that (!) of relativizer that (!) it is as likely to confuse the reader as help her.
Note, by the way, that even a subject relativizer may be omitted in casual speech:
There's this guy
who lives down the street from me who has a reciprocating saw I borrow to prune trees.
This should not be emulated in formal registers.