That depends on how much you want to reduce the unpleasant feelings of suffocation.
For the basic variant you can use hyperventilation and removing as much residual air from the bag and the lungs as possible. These techniques should reduce the time you'd feel suffocation and the overall time before passing out.
More advanced variants are based on using asphyxiant gases. Depending on the chosen gas, they imply a little or no discomfort. There are two different strategies:
1. maintaining constant flow of an asphyxiant gas without sealing the bag (like in the exit bag method) and
2. filling a large bag with an asphyxiant gas and sealing it after putting the head inside.
The first option may be done either with a gas cylinder filled with an inert gas or chemical reagents producing an asphyxiant gas (for example, CO2).
The second option doesn't need complex equipment, requires a lesser amount of gas than a classic exit bag, and allows usage of flammable or moderately poisoning gases without imposing high risks on other people.