We've been working on a ton of brakes in the last couple of weeks. All of the sand and salt that's been on the streets really does a number on brake pads. Mix that with rain and slush and you've got a perfect slurry for wearing out brake pads. If you've noticed that your brake levers take a little more pull to engage, you're in good company.
If you want your bike to stop better (and you probably do), your bike needs either a brake adjustment or brake pad replacement. Here's how to tell if your pads need replacing-
You don't need any tools, or even to remove the wheels to see if you need new pads. Just take a look. Brake pads have grooves that run top to bottom. This allows for water and junk to be cleared from the rim while braking, and they also provide wear indication. If you don't see those grooves- the pad is worn out and needs replacement.
You can see that the two "Partially worn" guys in the picture are more worn out on one end than the other. That's not uncommon to find (brake pads often need to be angled to stop brake chatter and squeaking). In that case you'd want to think about replacing those pads when any one of the grooves disappears.
Don't be alarmed if it seems like you're blowing through brake pads this time of year. It's (probably) not your fault (I don't know... maybe you're brake crazy or something). It's the weather. Just replace those pads and remember that spring starts on March 20.