Fabric is another great tool when it comes to dating a vintage piece. As well as certain textiles being fashionable (for example, Rayon in the 40s), some fabrics weren't even around until relatively recently.
Of course in order to be able use fabric as a clue to dating, you have to know what it is. Content labeling in the US became law in 1960 with The Textile Products Identification Act, which required clothing to state the percentage of each fabric used, but this only applies to commercial products.
So if you find something that's pre 1960, home sewn, or simply missing it's label - you'll need some other tools.
This website is great for identifying fabrics, weaves and finishes by sight and touch:
And if you want to be certain - you can always do a burn test.
Once you know what the fabric is, check out these links:
If your garment does have a fabric content - remember that google is your friend! A lot of companies used new man made fabrics as selling points, and mentioned them in their advertising.
Search the Google News Archives here. This is also a great resource for hunting out companies and brand names.
Care labeling became law in 1971 - but again, just to confuse matters, many companies were already labeling their clothes.
Dating vintage clothing is not easy, and certainly not fool proof - you have to use all the tools you have to pin point an era. For every single rule there's an exception - and often times even experts aren't going to agree.
But if you slowly and methodically take advantage of all the resources, you're more likely to come to an accurate conclusion.