I know that this was probably written about in official guides and landing pages, but I'll do this anyway so more people know about this.
There's a reward calendar to the right (click on your avatar), other than getting daily ARP you sometimes also get ARP boosters, and to use them you need to activate them beforehand. Most of those boosters only work for 48 hours, so do your calculations and use them the most efficient way you can. To activate a booster (provided you have one) you need to visit this link, bookmark it if you want. There you'll see all boosters you've ever gotten and you'll be able to activate them (if you click on a booster and there's no 'Activate' button at the top left corner, then it has already been used and/or expired).