What you wear depends on where you're going, i.e., work, pleasure, or other.
I'm retired now, so I don't have to be concerned about riding to a work location and can avoid Los Angeles' rush hour traffic.
When I ride in hotter weather, I used to wear a cooling vest I bought from Cycle Gear for about $30 to wear under my Tourmaster Air mesh jacket. It worked ok, but you ended up wet and then it would re-wick with your sweat and ended up stinking to high heaven. I also used a cheap hydration backpack I bought from Costco.
I then bought two versions of the Veskimo cooling units, used, from one of the guys on this site and have been very happy with it. There are two types, both of which use the same cooling vest with nylon tubing that allows ice water to circulate around your chest and back. The first type is a self-contained hydration pack that you can use to both rehydrate and keep cool while riding, and the second type is an insulated ice chest. Both types contain electric pumps to recirculate the water through the vest. The hydration pack is probably good for 1-2 hours before the ice melts and you need to stop to refill with ice. The ice chest is good for approximately a tank of gas and great for longer trips. Although an expensive solution, the advantage of the Veskimo system is that you don't end up wet, and the battery operated hydration pack can be used in a variety of settings - bicycling, gardening, hiking, motorcycling, etc. Both Veskimo units can be either plugged in to your bike for power, or operate with a lithium battery pack.
I also just bought a wicking undershirt at Costco and tried it out last week, but it really wasn't hot enough out for me to see if it was any good yet.