How do I measure the meter of a poem?

A poem's meter consists of a two features: how many syllables there are in each line and the metric 'feet' within each line. Counting syllables is as easy as it sounds:
'The time of year thou mayst in me behold...'
In this line we clearly have 10 syllables. Easy right? The trickier part is determining the metric feet. The best way to do this is to speak the line out loud and tap each time a particular syllable is emphasised. In this particular line, stress is placed on every other syllable. We call this an 'iamb' because it consists of one unstressed syllable that comes before a stressed syllable. Because there are 5 iambs in a line of 10 syllables, this meter is in 'iambic pentameter'.