I'm not that tall to begin with, combined with a very old front seat which is very saggy (the vinyl is in good shape) I am sitting pretty far down in the car. It would be nice to be where I was meant to be, which I think is a few inches higher when the seat was new.

Is there a simple/cheap/easy way to shove some life into the old seat, or is it one of those things to leave to an upholstery shop?

I would think you have a stretched out spring or two or possibly broke.
Seems it would be a fairly easy fix if ya could find new donor springs.
Probally wouldn't hurt to get some input from a upolstery shop

What I did, kinda redneck I guess, was the following:

Disassemble seat. Remove vinyl, foam, burlap.
Look at springs. Mine were in tact, just old.
I welded rods across the springs to make them move as a unit. Think of taking some coat hangers and tacking them to the springs. That's what I did. Raised the seat a bit, but firmed it up nicely.

Thanks for the ideas! I felt under mine this morning and the foam is crumbling through the bottom. After you added the wire across, did you add more or different foam?

I can't help wonder if cutting up a camping pad and adding it where the foam is/was in layers will help.

I think I reused the old foam on that one. However, when I can, I buy new foam. And yes, cutting up the green foam from a fabric store will help. The green foam is more dense. It is expensive. But then, so is this hobby! You can strategically place green foam under the factory foam for lateral support for your rear end too.

I would buy new burlap, that will help contain the crumblings from the old foam. I always use new burlap, and the thin cotton material when I can.

FWIW, I feel like I gained a few inches when I recovered my buckets. I used new wire, slightly larger diameter than original (0.055" I think), for the wires woven through the burlap. With new burlap installed, this tied the springs together into a nice firm base. On top of that, I used new foam from Legendary. The new foam seems to be much more dense than the original. No more butt sink.

Two things I did that firmed mine up alot:

Go to Menards/Lowes/Home Depot and buy carpet samples. The stiffer the backing the better. They are usually about 1.5' by 2.5'. I paid 4 bucks for two. Take your seat out, flip it over so you are looking at the springs, work the carpet samples under the springs between the springs and the foam. This keeps the springs from going into the foam.

If you are willing to take off your seat covers: I bought 1/2 or 3/4 inch thick high-density foam (green, if it matters) and used spray trim adhesive to glue this onto the top of the foam where you would be sitting. It was interesting trying to get the covers back on, they were very tight.

So there you have it, a couple of low buck ways to stiffen up your seats, these worked great for me....good luck.