The first style I learned was Moo Duk Kwan, but it was alongside TSD, so when I began to learn chung do kwan tae kwon do. I received a 3rd dan in Moo Duk Kwan before switching to start over at CDK. At the time, I felt I could not learn the new style, if I practiced the old, and so swore off practicing Moo Duk Kwan until I had reached a first dan and teaching status, and it was of comparable ability, or better than my original style.
I went 3 years without doing a single technique from MDk, but I found that when I tried to do it again, it had never left me, and perhaps coincidentally, kept growing alongside me.
I have a standard I try not to deviate from; as a 3rd dan, my goal is that every technique I utilize, be perfect in all aspects. If it isn't, then I train, and train, and train it until it is of comparable skill with my other techniques.
Just keep in mind martial arts is tied to us all our life, even in the times when we don't practice. You can take off a belt, you can't stop being a martial artist, from what I've personally come to find. People seem to be in a hurry to perfect a move, when they learn it. Whenever I learn a new technique, I earnestly practice it- but I also look forward to when I'm 70 (if I can make it that far) and appreciate my abilities will have probably improved by then as well. You can do a repetition of ten roundhouses twice a week and over time improve, but you could also do 10 roundhouse once a day, everyday, and improve even faster. In that comparison, I'm assuming one is executing the technique correctly... otherwise as we all know you're just reinforcing bad technique. Likewise, if you execute a single technique, and use it just once a day, within a decade chances are quite good it will be a very skilled technique. It's all a question of how fast you want to learn that other style, and in addition to learn, how to incorporate it into one's natural fighting style. Just remember that speed can risk sloppiness.
It brings to mind Bruce Lee's addage, 'I do not fear the man who has used 10,000 kicks, I fear the man who has used one kick 10,000 times'. I'm patient enough to practice what I know I need to, when I feel like it, and on other days, work on other areas. It's all about patience, and if you try to rush cross-learning different styles, you'll begin to actually mix the styles up into something new, and probably ineffective.
It was always important to me, and will continue to always be, that if I use a technique, I know what it is for, against, why I used it, and why it was the best choice in that context, in addition to being able to explain how the mechanics of that technique operate, and what style it's from.
Shotokan, Tae Kwon Do, and Muai Thai all have something which could be, and often is considered a 'roundhouse, or roundkick'. When you can differentiate between each, and utilize, as well as teach, I'd venture to say one has effectively cross-styled without murkying up their own art.
You can do a repetition of a strike until it's engrained in your memory. I feel... that by doing say a repetition of 10 on whatever I believe needs work on, at least once a day, allows one's body to also grow and change with the art, so that it becomes more natural.
It's why I can block in the 5 different ways of tkd, at the best time possible. Just... don't make the mistake I did, which was to think an art is 'crap'. No martial art is, and if there are any crappy ones, then they aren't of the art in actuality.
But also don't make the mistake that every art is complete. Not every art is a right fit for everyone, and certain arts compliment each other. Moo Duk Kwan and Wing Chun, Muai Thai and Krav Maga are examples. And the combination to best compliment changes from person to person, because we're all different as the cliche goes.