how do you stay warm in the winter when communting and how do you stay cool in the summer?

layers.

sorry for the short reply - yes, layers, but they're more obvious for the winter. layers are important in the summer too - with a good base layer you can stay cooler than if you were just wearing a jersey.

lots of people have recommended Craft, i have one cool-weather base layer from them that's awesome, but they're a bit expensive. others have pointed out that you can get good non-cotton layers at walmart/kmart/target/whatnot.

Read through the posts in the Commuting forum for advice.

Wearing bike-specific clothing helps you stay cool in the warm months.
Layers in the cool months keeps you warm.
Cotton holds moisture so you feel hot and stay hot in the warm months. For the same reason (feeling wet/damp) you feel cold in the cooler months - so avoid cotton winter and summer.

The best resource I know of for cold weather cycling is https://www.icebike.com/. Everything you could possibly want to know about winter cycling.

Clothes are a really useful way of keeping warm. On a bike the wind speed is higher but your workrate is quite high so you need slightly different clothes to pedestrians. Generally you need to wear less so you are a bit cool before the ride. It is important to take some extra insulation to keep warm when fixing mechanicals and flats.
Windproof outer layers are vital but waterproofs are needed in the rain. Extremeties need protecting and all gaps should be sealed. Synthetic wicking garments are good but many of us are returning to wool for a much more comfortable ride. The great unsolved problen for winter riders is footwear.
In the summer, the increased windspeed on a bike increases the rate of cooling so heat is less of a problem than it is for walkers.

On one bike forum I frequent, it seems that most of the people (in the USA) who commute, live in the northern areas of the country--where it's coldest. They often claim that riding is easier to do year-round in the cold than it is in the heat, because you can dress for the cold (adding or opening/removing clothes as you need while riding) but there's no really good way to keep cool in the heat.

I know I live in central Illinois right now, and on days when it's ~100 degrees and 90% humidity, extended riding isn't very practical or fun. Doing anything outside--even just standing in the shade--causes you to sweat enormous amounts.
~

Having moved from Texas, where I commuted in as high as 110 degree weather, the best advise I can give is stay hydrated. I mean, you can only wear so few clothes.

As far as commuting to work, go early, before the sun gets things hot. Take it easy, it's just a commute so no need to hammer.

Save that for the way home. Make it your work out.

It worked for me. YMMV

In the winter you layer and generate a tremendous amount of your own heat. I have biked down to -2x F and still not needed a lofting insulator like down or Primaloft. Really, it's amazing. The first block or two I'm usually wondering if I'm under dressed.

In summer, I've found a very light colored shirt (like UA) works best and I'm comfortable in a heat index into the low 100's F. Also, the fitter you become the better your body becomes at regulating it's own temperature.

The Winter Cycling forum has tons of advice on what to wear in the cold. It's very easy to stay comfortable in cold weather, as long as you are moving on the bike. It's not so easy to stay cool on the bike in the Summer.

As mentioned, cycling generates heat. It will never generate cool air, at least not enough to cool you on a very hot day. Your best bet in the summer is to still wear a light wicking jersey and cycling shorts and to stay hydrated. Another great tip is to invest in a Halo headband because sweat in the eyes is one of the most uncomfortable parts of Summer riding. The Halo pretty much eliminates that.

In theory, you should never be cold when riding, if properly dressed for conditions. Whether you buy the fancy cyclist clothing or wear things you already have, keep some notes on what you wore in various combinations of temperature and wind. The notes will serve as a starting point to help you decide what to wear when the temperature changes suddenly and drastically, or a year into the future when the weather cycle begins again.

Regardless of the season, your body adapts after a couple of weeks so that you can eventually withstand more heat or more cold than you first thought possible. But, it is necessary to get out and experience the weather to adapt to it.

I've found that it's the same as for hiking: no cotton clothing, ever, and make sure you eat and drink enough. That's the biggest issue for me, personally. I get cold very easily if my "fuel tanks" are low, and I can also overheat more easily (though hydration becomes the biggest factor in warm conditions, food counts too).

I am becoming a fan of arm and leg warmers for biking in in-between conditions (cold mornign or whatever) but I fully realize how silly my homemade ones look

Fancy cycling togs can be expensive. Winter clothing articles with similar characteristics, like wicking, can often be found for much less at the sporting goods department of discount stores.