I would recommend either a coach or playing against a high-level bot. Both will help you understand the opening that you're playing and the best moves and counters up to a point. Then after the games, I would try and play them back in memory and see how correct you are. But at our level and for a while, you don't need more than ten moves and a few lines since most people in our rating range tend to leave theory by move 3-6. A better thing to learn would be tactics and practicing principles. And theory will come with time spent playing since you should review all of your games and find the best moves for the next time.
studying theory

I find it much easier to learn/remember theory when I play over the board/tournaments and annotate my games. Playing longer time controls also allows my brain to remember what I am playing. It's true that most players at 1400 don't stick to theory, I would say that most players at 1800 don't either online. But all players in tournaments play serious openings. I recently played a 9 year old 1000 rated (USCF) OTB, and he played The French Tarrasch variation at 97.5% accuracy until the endgame. So lower rated players do know theory when they play in tournaments/OTB (At least the young ones do) Also, if you don't have alot of time to practice, try learning simple systems. For example, if you play 1.e4, instead of trying to learn every system of the open Sicilian (Dragon, Najdorf, Shevsnikov, Sheveningan) just learn one system that avoids all of those (Smith Mora Gambit). That's my 2 cents.

Whole games. Well-annotated. Mix quantity and quality. Sometimes I’ll search a particular position in my database, and then spend an hour playing through the moves from that position in as many games as I can fit into the time.

Step one: Get a good chess coach (Subtle plug, I'm a chess coach, more on OJChess.com)
Step two: Feel free to learn simple openings! I do the scotch gambit, a fairly simple one with lots of attacking ideas. It depends what you do at your rating, lots of times gambits can really be punished at higher levels.
Step three: REVIEW!!! This one is really important. Play out the lines the computer suggests. A fun game I do with my students is I have them try to evaluate a random position. Compare it with the computer, and see why you think the computer is arguing what it is. This is especially good with an experienced coach as they can tell you their reasoning as well as the computer's
Step four: Endgames! Learn simple endgames (opposite colored bishops is pretty easy) and ramp up to harder ones, with rooks, queens, and my favorite: knights. Learn how pieces interact, where the ideal spot for a rook would be. Another game I play with my students is I have them place down 2 or 3 pieces with their pawns and king and play from that endgame.
Step five: Play! Test out how your ideas fare against real opponents!
How do you get into studying theory? I know I should probably study it more but I never really have the motivation to do anything other than staring at an opening or endgame position for a few minutes, and I forget pretty much anything I do learn. Does anyone know any ways to learn theory and remember it?