The rule is, if your hypothetical scenario suggests something that isn't true, use were:

>If I were stronger, I'd break your arm!

(I'm not stronger.) 


>If I were a flower, I'd go crazy! 

(I'm not really a flower, though I've been called a pansy before.) 

>If my room were clean, it would be a first. 

(My room isn't clean.)

If it may be true, use was:

>If this answer was poorly constructed, it may have been due to the fact that I was really tired. 

(It may be poorly written.)

The same rules apply to sentences with *though*:

>He appears as though he were homeless. 

(He's not homeless, yet, although keeping up his current dressing habits could result in such.) 

This is addressed, among other places, on pages 56-57 of my favorite reference book, "Woe Is I" by Patricia O' Conner. 

Also see
http://m.grammarbook.com/grammar-rules/subject-and-verb-agreement.aspx from which I quote:

>Rule 10. The word _were_ replaces _was_ in sentences that express a wish or are contrary to fact:

>_**Example:** If Joe **were** here, you'd be sorry._

>Shouldn't _Joe_ be followed by _was_, not _were_, given that _Joe_ is singular? But Joe isn't actually here, so we say _were_, not _was_. The sentence demonstrates the **subjunctive mood**, which is used to express things that are hypothetical, wishful, imaginary, or factually contradictory. The subjunctive mood pairs singular subjects with what we usually think of as plural verbs.

>_**Examples:**
>I wish it **were** Friday.
>She requested that he **raise** his hand._

>In the first example, a wishful statement, not a fact, is being expressed; therefore, _were_, which we usually think of as a plural verb, is used with the singular subject I.

>Normally, _he raise_ would sound terrible to us. However, in the second example, where a request is being expressed, the subjunctive mood is correct.

>Note: The subjunctive mood is losing ground in spoken English but should still be used in formal speech and writing.