The 100 to 100 rule.
Things come and go, seasons change, and people tend to follow suit (as they should) because change is necessary. But with that said even while in a relationship *change* is also inevitable BUT That should not mean that the love you both share should become dry like a desert in fact it should be more like a plentiful oasis within this barren world. The key to this oasis is keeping the LOVE equal nobody should feel like they are giving more love than they are receiving, so in an effort to combat this both parties must in fact give 100% to each other, I'll call this the 100 to 100 rule.
Now these numbers may fluctuate given the pitfalls and barriers that tend to hit relationships but that does not mean it should ever deplete heck in all honestly if it dips below 50% you are already giving too much and settling for way less than you deserve ( 50 to 150 is extremely lopsided and a relationship is not a place that *lopsided* should ever prosper). Simply put keep the levels in a relationship as close to 100 to 100 as humanly possible and you will experience a love like no other. Love takes a lot of work and as you first meet the levels will be low but that is why you BUILD together until finally reaching 100%.
Both parties just have to be willing to do the work.