So, it's Friday and you're going to start low-carbing Monday. Excellent! If you're not accustomed to a low carb diet it's pretty hard to stick with it more than a few days, in my opinion. More than the first few days, that is. It definitely gets easier the longer you do it; after I'd spent six months on low carb (low enough to be in ketosis constantly) I found that I could spend a few days eating carbs again and easily get back into low carb mode. Those first couple of weeks were a struggle, though, so here are suggestions that I think are key to getting started.
First, one of the easiest excuses for dropping out of low carb is, "I don't have what I need to make low carb food right now." Plan ahead to avoid that. Go to RuleMe.com and AllDayIDreamAboutFood.com and /r/ketorecipes and all the other great places to find low carb recipes. Find some that look good, and go out to get the ingredients NOW. Get eight blocks of cream cheese. Get meat. Lots of meat. And eggs, and seasonings. Stock up on keto-friendly cooking stuff now so you aren't caught unprepared later."
Next, figure out your favorite salad stuff and again, stock up. I eat a salad with baby spinach leaves, ranch, feta cheese, and some kind of protein (turkey bacon bits, shrimp, or chicken) at least once per day and ideally twice. It helps you feel full so you don't get hungry between meals.
Snacks! Snacks can be the bane of low keto. I found I really like the Atkins bars and some of the Atkins candy. They're really convenient if I'm in a hurry, and I can sneak them into the movie theater. Likewise, I like the store bought cheese crackers (Moon Cheese and Cello Whisps) and recently started making my own in the dehydrator. Have that stuff nearby so that when you have to snack, you can do it low carb.
Quantity in cooking. If you're going through the trouble of making a new recipe, make enough that you'll be able to eat for the next few days. I love the cheesy chicken and pepper soup recipe I found, and I make it in batches that will give me four meals. Combine that with having some other stuff already prepared, and I can go four or five days without cooking, but without relying on Atkins bars and cheese to get by.
Finally, find a tracking app and use it religiously. I started out using MyFitnessPal, but now I'm using Fitbit's app for both my exercise and calorie intake tracking. Which is good, since it easily shows me my calorie deficit. Most important, though, is that tracking your calories and carbs keeps you really aware of what you're eating. If your goal is to stay in ketosis you need to make sure your carb count is very low. If you're working on weight loss, you want to be tracking the calories as well. This was a big one for me -- after six to eight weeks of tracking nutrition to stay in ketosis, I found it much easier during my carb-ful breaks to stay in a good calorie deficit despite eating carbs.
Most important note -- keep getting back on the horse. Remember, it's a long haul, not a sprint. If you can sprint for a while, great, but overall, health is something you're maintaining over decades, not days. If you deviate from your desired course one day, just get back on track and try again.