I use this stuff :
Schwalbe Easy Fit Mounting Fluid 50ml
https://www.bike24.com/p25556.htmlIt works about the same as diluted washing-up liquid but is a bit quicker to use, slightly cleaner and a lot more expensive.
The sponge on the little applicator in the link is convenient, but tears off after a few uses. So I bought the 1 litre workshop bottle and use a paintbrush for quick application when mounting a new tyre.
Probably only worth it if mounting tyres often, at one time I was doing this about once every 3-5 weeks when doing a survey contract with a Brompton. Small tyres wear out faster.
With some tyres I have to deflate, run my fingers around the tyre on both sides and repeat the application. On the other hand, some tyre/rim combinations don't need the mounting fluid at all.
And if I am at home I place the tyre horizontally on a large plastic dustbin without a lid and inflate it in that position rather than vertically, to avoid any effect of gravity.
Be careful about overinflating.
With some tyre/rim combinations this works a treat and with (and sometimes without) the soapy lubricant the tyre just pops into place, sometimes audibly.
But going too high can destroy a rim, especially with fatter tyres, I've done this once with a
new rim (50 mm tyre inflated to 5.5 bars) so nowadays I don't exceed 4 bars when fitting 50 mm tyres and don't run them at more than 3 bars, usually less.