I work out 6 days a week, unless I'm traveling. That can vary anywhere from 3 days weights/3 days cardio to all 6 days on weights.
Sundays are my day off, so 5 of my 6 gym days are weekdays. I build gym time into my weekday morning schedule, which helps ensure that I get there. I get up early, eat a quick breakfast, and drive to work (where my primary gym is) ... if I am there at 7 a.m., I am NOT just going to go sit at my desk for an extra two hours, so that helps goad me into hitting the gym.
A benefit of morning workouts is that it really energizes me for my day, which also helps get me to the gym on draggy mornings: I think about how much better I'll feel later for having worked out (or how crappy I'll feel if I don't!) and that motivates me.
Also, I'm an "easy gainer" (muscle up very easily), and those very fast, and very apparent, gains also provide huge motivation to keep at it.
I do like changing my routine, and it's definitely important physiologically. If you do the same thing indefinitely, you'll stop seeing results. Currently I have about 8 lifting routines I cycle among. Some emphasize strength, some muscle mass, some definition, and some a balance of all three. Keeping it varied also helps keep it from becoming boring.
I also vary my cardio a lot. My mainstays are swimming laps, spinning classes (indoor, high-intensity group cycling), bicycling to work (13 miles one-way) a day or two a week in the warmer weather, and the elliptical machine, either by itself or with 30 seconds of jumping jacks and push-ups interspersed at two-minute intervals.
I also love a variety of outdoor activities (hiking, biking, kayaking, snowshoeing, cross-country skiing) so to the extent possible I mix those things into my fitness plan. Definitely helps keep it interesting!
Steve