First even if you’re not in a painting rut, join a painting group that has a monthly deadline for a painting submission. This can give you the extra push to get paint on your models, as well as a sense of urgency to have it done before a deadline. Also it is fun to show off the painted models to the group of other hobbyists who are doing the same thing. Be ready to give and receive constructive criticism, but that is ok, it will make you a better and faster painter in the long run. Also sometimes they will do a monthly giveaway for best painted model, or raffles for finishing on time with is a nice added bonus.
Second try something new so that you are not painting the same color over and over. Going outside of your comfort zone is a great way to keep painting fresh. Even if it is a unit of 10 or 20 miniatures, try a different color scheme or a new technique to learn something, and be excited about getting to those models.
Third is to switch it up and start a different army or game to keep the models fresh. Meaning if you just painted 40 pink horrors (that was a long month) then instead of painting more Chaos paint some models from a different faction. Or even a different game like Warmachine, Silver Tower, or some DnD miniatures, etc. Switching it up helps you get different paints in your collection, as well as freshen up your painting pallet in that you are not doing the same 5 colors over and over again. When you go back to more Chaos (or whatever army) you will have more colors too use in your paint box, and might even have some more elaborate painting schema.
Fourth is to switch between larger models and units. Painting 10 guys in a conveyor belt method can be tedious and get old pretty quickly. By rewarding yourself with a cool standalone model once you finish the unit you are giving yourself a goal to work towards and something to look forward to. Just be sure to finish the unit before diving into the model, otherwise you might never go back and finish.
Finally take a break and do something else instead of painting. Have a bunch of guys still on spruces, get them cleaned and put together ready for primer. Or have fun basing some models so you can play with different mediums (flock, texture paint, rocks, etc.) to get that final touch on them. Once you have taken a long enough breather the itch to finish those miniatures will come back and you can get the rest of the paint on them.