Are you having your tires flat so many times? Mostly the problem is caused by the spoke nipples at inner side of the rim. It will be no problem if you keep measuring the air pressure of your wheels all of the time, but we don't do that much, do we? When the air pressure is too low, the inner tube does not stick well to the rim nor the outer wheel, then the spoke nipples are starting to slice the inner tube, moreover when they are rusty.
Before you install a new inner tube in, let's make use of the old one to cover the nipples shall we ...
Step 1: Cutting
Measure your rim. Pay attention to the spread of the nipples. The holes are designed to be zig-zag so you need to measure the width from the left of one hole to the right of the other hole next to it. In my case they are spread about 1.5 centimeters leaving 0.5 centimeter from each side of the rim.
Measure 0.75 to 1 cm from the inner tube's side. When you spread the cutting it will be 1.5 to 2 centimeters. If you want to make an unbroken loop, start cutting from the outside and leaving 1 centimeter at the inside. Or you can also start cutting from the inside out. My old inner tube has a mend at the outer side, so I take the inner part. Unbroken loop is only usable if you are using the tube from a smaller wheel. If you are using your old inner tube for the same wheel, then it will be loose. But somehow you can see how I cut the unbroken loop from my photos. You can start cutting from near the valve.
One hand keep the tube fold neatly by the middle line. Go on cutting slowly. Centimeter by centimeter. Inch by inch. Do not rush. Patient is needed in this cutting work.
When you meet the valve, cut it side by side because that part is thicker and you can't fold it half.
If you lost your patience and you feel boring and you want to get it done in a rush, then it will end up like the photo above. You will have one side wider and the other side is narrow. It is okay when you have only small section of this. So, well-pinch the tube by the folding line and slow-cut will give you a neat cutting.
Step 2: Removing the Valve
Use a cutter or pen knife to slowly remove the valve. You might need a file on the thick part around the valve if you want to keep the unbroken loop. Adjust the hole so that the valve from the new tube can get through it.
Step 3: Stretch Around
If you find the loop too loose to get in the rim (if you are using old tube from the same wheel, it must be loose) then you can simply cut off the thick part around the valve rather than filing it.
Fold the tube into half, then cut a "V" shape about 1 centimeters from the end. When you open the fold, it will become a square hole.
Put your new tube's valve ,through the belt's hole, to the valve hole on the rim (well, assuming that it is the new tube's valve. I have not buy a new one yet :D )
Circle the belt along the rim, covering all the nipples. Cut the excessive belt at around 1 centimeter from the valve, that is at the other end of the belt. Make a hole at 1 centimeter from the end of the belt using the above method (fold into half and cut "V" shape).
Pull out the new tube's valve. Pull the belt and let the hole in and then put the valve back into the rim's hole. Adjust the belt around the rim so that every nipples are covered well.
There it is. The belt that saves your new inner tube from being sliced.