Yep, I'm bipolar and lithium gives me essential tremors, I have a few ideas.... 1) Brace your arm against something while filling, and if you can, use needle-tipped bottles for greater precision-- like, brace the hand holding the mod/tank and the one doing the filling.
Also, with bottom fill tanks that may come apart if not held together for filling, packing tape works to keep everything steady if it's a bottom fill, for example, wrapped around the top of the tank and the glass, to keep them from separating.
If you can, get tanks that are easygoing to fill, and leave you a wide space for filling, and regardless of whether a tank is top-fill or bottom-fill, inspect both sides to see which looks easier for filling.
Keep a rag, a rubber mat under you possibly, to catch any drips or spills, and to wipe up afterwards.
There's no magic solutions for this, so stay calm. I can actually build some RTAs if I work hard at it, and pick the right time of day (my tremors worsen as the day goes by, so I try to fill and do whatever and whatnot earlier in the day if possible).
Don't freak, and maybe consider DIY if you are worried about juice wastage, as a certain amount may be inevitable.
Best of luck, and I hope you get a system going. I've found practice and patience (and creativity) to be my best friends as far as this goes....
Anna