I don’t know that this is necessarily a problem unique to freelancers. Everyone has the same “work, work, work” mentality most days and actually planning for time off is hard.
But for freelancers in particular it can be exceptionally hard for a couple of reasons. First, free time is never guaranteed. Second, planning for free time involves not only planning your trip or days off, but manipulating your schedule and income to support it.
There are a lot of moving parts, which makes it very hard to do, especially when you are already busy, and double especially if you have children or your spouse works a normal day job. Here are some tips to help overcome these issues and ensure you stay on track for your next big vacation.

Set a Time Frame Far In Advance

Last minute vacations or time off are hard to pull off if you’re a freelancer. Really, they’re hard to pull off for anyone, but the mental image of a freelancer jetting off to Europe for the hell of it is a romantic notion that is rarely plausible.
The exception is if you have some money set aside and plan on working on your trip. Since the biggest obstacle of time off for a freelancer is project and client management, this probably won’t work.

Communicate with Regular Clients 1-2 Months Out

Tell all of your regular clients that you will be away at least a month in advance. By giving them extra time, you make it possible to get any recurring work for that period done in advance.
I did this before taking a two week vacation a few years ago and, while I had to work 12 hours a day for two weeks, I was able to do a full month’s worth of work for most of my clients in just two weeks. It paid for the trip and then some. Alternately, some clients may be okay with extending deadlines until after you get back.

Distribute Extra Work Over Time

Time off means time not making money. Freelancers don’t get paid vacation (unless you have a nice surplus in a business account), so start adding a bit of extra work to your schedule a few weeks early. Two months before my son was born in 2011, I added a sixth day to my schedule. I worked a total of 9 extra days in two months before he was born and I needed it. For about two weeks after he was born I did very little work, so having gotten ahead was very important.

Line Up Projects for Your Return

The worst feeling in the world is when you line up projects for weeks or even months in advance of a trip, have plenty of money socked away, and then go on your trip, only to return and have nothing on your schedule.
You planned for two weeks off but end up taking a full month off because you have an empty schedule two weeks after returning. Talk to regular clients, put out a few bids and set up a few things to do for when you get back. You may even want to devote one day in the final week of your vacation to sending query letters or bids on projects.
If you do it right, your next vacation can be as simple as it was when you worked a day job. Hiccups may occur and you might need to logon for a few minutes here and there, but the goal is not complete detachment (you are the boss after all); it is freedom to enjoy your trip. If you can do that without it hurting your business, you are doing something right.