If we make ROI as our KPI, then we should know what is an ROI.
ROI = Revenue(Profit)/Spending
So if we want to make ROI positive, we have to enlarge revenue, and at the same time lower the spending.
I think there are several ways can help you reach that goal.
1. Buy CPI-mode traffic.
It's obvious that the cost of install can be controlled by this mode other than CPC or CPM.
What effects the conversion rate of install?
a. Size of the pack
b. Click by mistake
c. Internet environment(wifi/3G/4G)
d. Ads creative is so different from landing page.
If you don't want to take a risk of convertion from click to install, just let ad networks to afford them.
2. Optimize your bid price.
I suggest that you should know well the average CPI bid of region that you are promoting in advance. Then you can start with a lower price. Never be tricked by the ad networks to give a high price.
3. Optimize your creatives and slogan before raise price.
When the impression frequency reaching more than 2, you can begin to change your creative. Or you can have an AB test to select your most working creative and slogan. Ads fulling of creativities really work!
4. Avoid big company's promoting and connect your product with trending event.
Actually, finding out your target user and give them what they want is the most effective way of saving marketing budget. So before every promoting, please do more work on your own product, to know it's features and superiority over your competitors.