I used to trade in both TF2 and then CSGO after. I consider myself very successful because I got most of the things I wanted and I just followed one rule:
ALWAYS TRADE FOR PROFIT
Never, ever, trade for something that is worth less. Even if you're desperate for an item. Don't offer more than it's worth because you will regret it in the long run. Be realistic though. If you can't afford an item, don't even try to trade for that item instantly. Work your way up to that item. Keep that item in your mind as your goal. Sure it may take a bit longer but you don't want a bad rep for low-balling. At the same time, be wary that the higher the risk, the higher the reward.
Always check out trade servers (the popular ones where the super rich go to trade as they tend to have more reliable traders). Keep an eye out on trade websites. I used to use 2 back in the day. Backpack.tf was one and there was another, better one but I forgot what it was called. (Sorry!)
Be patient. You may find someone with the item you want but they may ask too much for it. Just don't go through with the trade or you'll lose out in the long run and find someone else on trade servers/websites who are willing to give it at a reasonable price.
Someone always loses out on a trade between 2 people. Make sure it is not you. It is rare that 2 traders want the exact same items as each other.
After a successful trade, keep that trader on your friends list (unless they remove you) and add [Trader] as their nickname. They may trade items with you in the future and you know they are reliable.
I've retired from trading now and I don't keep up with the currency and whatnot but I can tell you for a fact that TF2 trading is easier to make profit out of than CSGO trading.