There are a few – mercifully few – verbs that conjugate irregularly in the subjunctive tense. However, not only do they tend to be verbs that are irregular in virtually every other tense (and so easier to remember as irregular), but they also follow slight patterns.
être: je sois, tu sois, il soit, nous soyons, vous soyez, ils soient
avoir: j'aie, tu aies, il ait, nous ayons, vous ayez, ils aient
A few verbs just use irregular stems, but otherwise conjugate normally:
faire => fass-
savoir => sach-
pouvoir => puiss-
And finally, there are a handful of verbs that have two stems that use an irregular stem for all conjugations other than for 1st and 2nd person plural.
vouloir => veuill- (but nous/vous voul-)
aller => aill- (but nous/vous all-)
valor => vaill- (but nous/vous val-)
A good way of thinking about conjugating subjunctive verbs is that it makes the last vowels sound. For instance, while in the present tense one says “il part" without pronouncing the “t", giving it a longer sound, in the subjunctive tense one says “il parte", thus pronouncing the “t" and giving it a more abrupt sound. While this way of thinking about it doesn't always work, if you're uncertain of how to conjugate then this will you get you in the ballpark.