Tech skill is like any skill. If you train hard enough, you may get rusty but you will never "lose" it.
Real world example: I played tennis for 2 years in high school. This spring I played for the first time in 5 years. I'm significantly more out of shape, but the fundamentals that I learned before I still knew. I could still serve fairly accurately, I could topspin and backspin.
To get this for tennis, I practiced 2-3 hours a day 6 days a week for months on end. We did many drills, mostly person vs person following certain patterns. Sometimes we used a ball machine. Sometimes we just played regular sets.
My suggestion for smash is to play someone who challenges you without being on a rotation. Just play that one person until your hands are sore, then play some more. Don't keep track of score but try your best to win. Do your best to stay creative; you want to be able to link various elements of tech skill together organically. That is, you want to be able to observe and react on the fly.
Practicing against computers without a specific focus (like pivot grabbing fox during marth's CG) will eventually cause you to get bored and start doing things automatically. The longer you practice during this automatic mindset, the harder it will be for you to do things dynamically against a real player. If you want to practice alone, have something very specific you want to work on. Plug in another controller or use the name entry glitch whenever possible.
I practiced alone a few times over the summer preparing for FC, when I did I was working on specific techniques to solve spacing issues I was having. I spent a while just shffling and moving around the stage normally, then I started to shadow box (i think thats the term) which basically means imagine a problem scenario and then practice various responses. I practiced spacing my dash dance and using shield-pivoting to allow me to control my bair spacing better. Then I put on a marth and practiced some basic automatic combos, such as dair->shine->wavedash->dash->usmash. I spent some time making sure I could usmash 100% of the time using the claw method (my opinion is that since I don't have to use the control stick to jump, I can better control the spacing/timing, especially during a quick dash dance).
Now you don't have to learn those things, but you should have a focus. If you don't know what you should improve on, the energy spent practicing would be wasted. If you need guidance in that category, i'm sure there are plenty of people you can talk to for advice. Ask your training partners, post on the boards, get in contact with the best player you know and just ask for advice. Watch videos and analyse them, don't just watch them for amusement. You can analyse any video, not just pros'. In elementary school I remember my class had to practice critically reading by using the "5 questions". Who, what, why, when, how. Its a pretty good technique if you get stuck.
some examples:
Who...
has the advantage?
won that trade?
What...
happened? (I still ask this a lot haha)
is their strategy?
Why...
did he make that choice?
did that strategy work?
When...
should he have grabbed edge?
should he have timed his attack to do better?
How...
did he know?
do they do _____ ? (this can range in scope from the actual inputs to how they form their tactics, etc)