When you ride your bike to work, you watch the weather more closely. Not that it helps in the beginning. For me it was a crap shoot- sometimes I did better than others. When there's a big change in the weather I sometimes miss the optimum. However, with some practice I've developed some guidelines that seem to get me close.
One thing I heard early on is that if you're not cold the first mile or two, you're overdressed. There's some truth to this. The exception is if you're riding in a stiff breeze. If you start out cold, it will take much longer to warm up if the wind is blowing through your clothes.
I dress according to the weather, and sometimes I will be able to wear some portion of my work clothes while I ride. I'm just now starting to get dedicated bicycling gear; most of my commutes have been with normal clothes. Here's what I've figured out so far:
60 degrees F and above:
Bottom: Shorts or short bike pants.
Top: T-shirt or a short-sleeve bike jersey.
If the temps are in the 60s or above, you will pretty much warm up instantly. If it's windy, you might want to wear long sleeves.
50 to 60 degrees F:
Bottom: Shorts or short bike pants.
Top: short-sleeve t-shirt and a long-sleeve t-shirt, or long-sleeve t-shirt and light windbreaker.
I don't use them at this temperature, but many people like to wear either leg warmers, tights, or long bike pants for temps in the 50s. In fact, one rider in my club swears I am doing terrible damage to my knees by leaving them uncovered at this temperature. Phah, I say.
And as for windbreakers, I have one of these (as mentioned in a previous post):
It's tailored for cycling and the more I use it, the more I like it. Like other cycling tops it's longer in the back than in the front, it has several pockets with zippers, and one pocket is actually a pouch that the jacket can can fold up and store in. Also the sleeves can be vented or removed with zippers.
40 to 50 degrees F:
Bottom: Jeans, or short bike pants with tights/leg warmers, or long bike pants.
Top: T-shirt, sweat shirt & light windbreaker, or t-shirt, long sleeve t-shirt, and sweatshirt.
Other: Light gloves or full-fingered cycling gloves, maybe a hat or pull up the hood from your sweatshirt under your helmet.
The first time I did the hood thing, I pulled up the hood and pulled my helmet on, resulting in a tunnel-vision effect; I had very poor peripheral vision. I find that I have to put the edge of the hood on my hairline and no lower, and buckle the helmet strap such that it holds the sides of the hood back, to get good visibility.
30 to 40 degrees F:
Bottom: Jeans, or long bike pants.
Top: T-shirt, long-sleeve t-shirt, sweat shirt & light windbreaker. Substituting a turtleneck for the long-sleeve t-shirt keeps your neck warm.
Other: Light gloves or full-fingered cycling gloves, plus a hat or pull up the hood from your sweatshirt under your helmet.
As the temp gets down into the lower 30s you may want to consider a face covering like a balaclava.
I wear glasses and so far haven't needed anything more than that for my face. But when I wear a balaclava, my glasses steam up if I'm stopped or moving slowly, unless I pull the balaclava down below my mouth. Otherwise my warm, moist breath comes up under my glasses and they fog up.
20 to 30 degrees F:
Bottom: Jeans with either tights or long underwear.
Top: T-shirt, long-sleeve t-shirt (or turtleneck), sweat shirt & light windbreaker.
Other: Heavier gloves (mine are fur-lined) or full-fingered cycling gloves for cold weather, plus balaclava. Toward the low end of the the temperature range a hat or hood may be needed with the balaclava.
I haven't gotten below that yet; the coldest I've seen here in Texas is 22 degrees F.
As a general rule, I don't ride to work if it's already raining. If I am caught at work and have to get home I'll go for it, figuring I can take a hot shower when I get home.
As for my routine upon arriving at work, I take a change of clothes with me. If I wore jeans on my ride, I usually use those for work. I take a change of socks and underwear and a work shirt. If I want to wear a sweater at work, I may use that for one of my layers when I'm riding. I'm lucky in that I park my bike in a closet that I can lock so I can change in there. I strip down, take a "baby powder shower" (I take a normal shower before leaving home), and put on fresh deodorant. Then a take a washcloth, hand towel and a comb over to the bathroom and rinse off my face, rinse out my hair, dry off and comb my hair. Pretty much no one can tell I've ridden my bike.
Because my bike is in a storage closet I can hang up my riding clothes on the backs of chairs in there and they dry off during the day. The best suggesting I've heard if you don't have anywhere to hang up riding clothes is to tightly pack them in a plastic bag and seal it. The only problem with that is you really can't use them for the ride home. You either have to wear your work clothes or you have to bring a third set of clothes for the ride home.
There is the fear of forgetting something, but it becomes routine pretty quickly. If you're worried about forgetting anything, maybe make a check list and/or get all your stuff ready the night before.