This could cause your clutch to lose partial traction, which wastes fuel and causes wear on parts such as your throwout bearing. Although, it depends on how heavy your foot is and how light the pedal is. At any rate, we suggest keeping your foot off the clutch pedal unless you're shifting gears. This manual car driving tip is worth mentioning every time the subject is discussed.
You could wear out certain parts (namely, the selector fork) too quickly by resting your hand on the gear shift because the weight of your hand and forearm could be enough to move the parts out of the places they naturally rest. Besides, it's a good idea to keep both hands on the wheel when you're driving. It might take a while to get used to obeying this rule. Manual car driving isn't always convenient!
Your clutch's bearings press against its diaphragm spring when the vehicle is in gear. If you're not going anywhere while the clutch is engaged, your causing a little bit of unnecessary wear on these parts. For the best conservation of your vehicle, put it into neutral when stopped at a light or elsewhere.
Your clutch disc will wear when you do this because it will be putting a great deal of unnecessary friction against the flywheel as the two parts are pushing against each other and rotating at different speeds. You should minimize this incomparable rotation as much as possible. Keep still on a hill by pressing the brake. If you're not sure you can accelerate without rolling backwards, pull the emergency brake and accelerate as usual. When you feel the car start to pull, release the emergency brake and you will begin moving forward. Practice this somewhere away from traffic.
If you're going 70 miles per hour and you punch the gas in 6th gear, you will not see a big jump in speed. The engine will struggle to bump it up even a little, rather. This is because higher gears sacrifice torque to maintain a high speed. If you need to accelerate quickly and you're already in top gear, shift down a gear and give it some gas to give the engine some leverage.
If you find this hard to understand, find a bicycle with several gears and put it in the top gear (you'll have to pedal to make it shift). Stop and then try to accelerate quickly. Then put the bike into a medium or low gear and try again. You'll notice a huge difference in the ease with which you can accelerate.
Not every driver knows these tips. Now you can consider yourself an enlightened manual car driving enthusiast!