Life can move slowly in Ghana. Embrace this! Walk slow, drink water and minerals, make sure you get some salt in the morning to stay hydrated, and take breaks as you need them.
When I first came to Ghana, I always rested during the hottest hours of the day (usually 12-2 pm). Meaning, I'd find some shade, sit back, and have something cold to eat or drink. Alternatively, I'd find an air conditioned internet cafe and spend these hours inside.This made it much easier to get through the day. A mid-day shower never hurts either. It was January when I arrived, so I had a little trouble adjusting to the 100 degree (F) temp. difference, and had swollen feet and heat rash for the first few weeks.
Also something to keep in mind, the day starts EARLY in Ghana, for some people as early as 4 am. I noticed a lot of runners and sports teams meeting up as early as 4 am or 5 am, and at this time, the weather is perfect for sports activities.
About buying clothes, and re-emphasizing what was said above about buying your Western clothes before you go, I would say don't plan on doing too much clothes shopping in Ghana except for the locally made clothes (shirts are a great thing to get made, and very cheap). You will probably have more important things to deal with the first few weeks than rummaging through the piles of used pants and bargaining. There are also clothes stores (with new clothes and prices comparable to those abroad) and more organized shops where you can buy used clothes. But figuring it all out when you first arrive might be a bit frustrating. Even more, Ghanaians are used to the heat, and aren't as concerned with lightweight fabrics, so you will have to dig through these stores to find the lightweight clothing you need.
Last of all, when you just can't take the heat anymore, hit up the beach. A swim and a cool sea breeze will be a big relief.