Concentrate on collecting coins. They are used to purchase upgrades. Your survival chances will improve after you get a few upgrades and the second character. He has a better energy power-up. After unlocking him, get three more upgrades.
Continues
You can watch a video advertisement to continue your run. Do so only if you were on a good run, as the number of ads that can be viewed daily is limited.
Layout
Focus on the top of the screen to collect coins and energy bits dropped by some bombs easier. You will be able to see the patterns that the incoming bombs create, and be able to avoid them easier. After things get crowded you will be forced to also consider the bottom half of the screen. Remain towards the middle to avoid getting pushed to the sides and getting trapped. When you see a pattern in the bombs that allows you a way to squeeze between them, use it to stay in the middle.
Power-ups
Collect the power-ups that drop randomly. The more you unlock, the more power-ups there will be. After purchasing the first two power-ups, save your money for the extra heart power-up. This will allow you to play longer, resulting in more coins and rewards. You can also unlock new characters that have different abilities. After finding one that suits your playstyle, upgrade them as well.