^^^^^^^^^
Something skinny, like a curb, 2x4, or rail.
For a while my “sinnies” were that 5" wide or so painted line on the side of roads and in parkinglots:p I still can’t do anything narrower than a curb:(
I think it’d be a good idea to nail/screw on some stabilizers when riding a 2X4 on it’s end.
I can only still stand for like a second, but while lightly holding onto something, for me it’s not easier standing up or sitting down, it’s just different. While standing it seems as if the fulcrum is the hub/pedals and while sitting it’s as if it’s the seat. While sitting it’s a bit easier w/ my seat 1-1.5" shorter than my normal height.
Edit: Now that I think about it, still standing for me is a bit easier when sitting but nearly 100% of my weight on the pedals.
No need to 
… I learned to ride skinnies by riding the curb on sidewalks.
Painted lines don’t do as well for skinny training as there is no consequence to riding outside the paint. With curbs, you’ll get some definite feedback as to when you’re over the edge.
The reason you need feedback is to find the limit in which you can ride a skinny. Even when riding on the skinny side of a 2x4, you’ve got a bit of “wobble room” in which to ride. You need to find that wobble room and let your muscles learn how to use it… takes time.
Once you learn to relax and trust your wobble room, you may find skinnies much easier. For me, skinny riding is 75% mental and 25% physical skill.
I have a bunch of narrow logs nailed to pallets in my sideyard:)
The ones that are really fun are the ones I didn’t cut the knots out of. Quite a bumpy ride along a 2-3" (yes, wood tapers :p)round, slightly twisted skinny that’s angled from a 4 pallet stack to 1 pallet.
i find it easier with lower seat because you can move your knee to help balance. ive done a 3cm largeX10’ long skinny with really low seat.