The trouble is that for a lot of us, "write every day" feels more like a cruel joke than a serious piece of advice. In every week, I have exactly two hours and fifteen minutes without a child under six in the house, and part of that is taken up by little things like personal grooming and making sure the electric bill is paid. Before the kids were born, I was a teacher, and it was hard to find writing time when I was grading essays and midterms. When I couldn't write every day, I'd get into a funk--because writing every day is what real writers do, and if I don't, I'm not a real writer, right?--and eventually, I'd end up not writing at all.
If you're in the same boat, it's OK. Even if you don't write every day, you can find a writing schedule that works for you. I've gone from writing a short story or two every year to hammering out a rough draft of a novel in a year, and it wasn't because I suddenly got more free time; it's because I figured out how to write regularly without writing every day. This is what worked for me; if it works for you, go for it.
*Set a weekly quota, not a daily one. I'm familiar enough with my writing process to know that if I set a really high bar and fail to meet it, I'll give up, so my initial weekly writing quota was one page per week. (Laugh all you want, but it was better than I was doing at the time.) When I got comfortable with that, I gradually upped it. I've worked myself up to a weekly quota of six pages--about the same as a page a day with one day off per week, which was what I was trying to do in my "write every day" period. The difference is that this time, I'm actually doing it.
Conversely, you might find that a high bar encourages you to jump that high; in that case, setting up a page-a-week quota is just giving yourself permission to slack off. In that case, great! Aim higher.
*Schedule writing time every week. Seriously. Look over your schedule, find a time that you're generally free, and set that time aside for writing. It can be just a little at first; the important thing is to make it a habit. I meet a friend after church every Sunday to write. It felt like going out of my way at first, but now it's such a regular part of my schedule that it's weirder when I don't write.
*Have some way of holding yourself accountable. Like I said, I meet with a friend who's also a writer. It helps; if I skive off writing one week, I'm standing her up, so I only cancel when it's important. On Writing Excuses, Mary Robinette Kowal has mentioned having writer dates over Skype, which could be helpful if you have a writer friend who doesn't live nearby. You could also try emailing a friend after your bloc of writing time to let her know how much you wrote that day, or using a productivity app (I love Habitica for this) that rewards or pings you depending on whether or not you write. If you have the willpower for it, you could even say that you aren't going to watch your favorite show or buy that new board game until you write a certain number of pages; I have never managed to make that work for me, but my husband has a huge pile of board games that he's gotten as rewards for completing various tasks, so to each their own.
*Remember that writing a sentence a day is still writing every day. It sounds stupid, but it's true. If you write a sentence, you've written. You're keeping the story fresh in your head, and putting yourself in touch with your narrative voice. When I sit down for my Sunday writing session, I have a much easier time sliding back into the story when I've written as little as a paragraph or two in the preceding week. What's more, if you write a sentence, you may find yourself writing another, and another, and another . . . or you may just have time for that one sentence before you get back to the daily grind. But it's a sentence you didn't have yesterday.
Of course, this isn't the only way to get yourself writing more, but it worked like a charm for me. I'd love to hear about other people's stories. How did you get yourself to write more often even when you were swamped?