Actually, you lose LP based on your MMR in comparison with your current ELO (Division/Tier).
If you play a game and get S-/S/S+ then your MMR actually has no negative effect, save for maybe a few points lower.
It may look like you're actually losing ground due to the loss from the game, but in reality you're not losing anything at all because the next game you queue into will have the same MMR or higher due to your performance the last game you played with the S-/S/S+.
MMR is calculated by a LOT of different factors, and how well you play in each game is a huge factor for gaining MMR.
TL;DR - Just keep playing each game to your best and you will likely win more than you lose, along with gaining lots of MMR.