It's an air cooled engine, therefore it relies on the air to cool the engine via the cooling fins.
So anything that prevents air to do this can cause the engine to run hot. As stated above, avoid long stops (red lights, stop signs, etc). Use the provided kill switch at such points. It's too easy to peddle 3-5 times and drop the clutch and start that baby back up. It's an internal combustion engine, it's going to get hot, there's fire in there, and on top of that the metal is cheap Chinese metal. I'm sure a small mounted fan near the engine could prove to help, but not much.
Don't be too scarred that it gets hot. Ever feel your car engine after a drive? Try it. And thats liquid cooled (radiator).
On the statement regarding riding it downhill, if it is a large hill just pull in the clutch and kill the engine. Why do you need the engine to idle or run while going downhill? Especially if you just let it idle, might as well kill the engine and let it cool.
Your gas/oil mixture does matter, but so does your fuel/air mixture. Check for leakes around the carb using brake cleaner. If the engine dies when you spray it, then there is a leak (dont spray on air filter). A lot of people get confused and think the gas/oil mix controls running rich or lean. It's actually the air/fuel mixture. The more air that gets in, the hotter the engine will run.
However, be sure to properly lubricate your engine. Personally I use 35/1 as recommended by thatsdax for breakin and normal operation. It works good for me thus far. But you have to find what works for you. Too much oil fouls your plug and oil will build up in your exhaust. Not enough oil and your engine will not be properly lubricated and can cause engine failure.