Hello Adam,
Two points, the first and easiest is the idling speed of the engine, too high and it makes engaging first or reverse difficult. An average idle would be about 700 rpm for most British classic cars.
Secondly, some clutches are self adjusting for free play at rest, but not all, worth a check, There should be about 1\16" to 1\8" free play at the slave push rod to actuating lever.
However it sounds as if your synchromesh is weak, quite common really. By driving differently you can compensate for some of the faults you describe. Jumping out of gear is the one you can't compensate for and requires some attention. If you are lucky it may be the body fouling the gearshift as someone suggested, otherwise it is a gearbox defect.
Bearing in mind that it is possible to drive and change gear without using the clutch at all, puts the importance of the clutch into perspective, i.e. not really that high.
To ease the changing process; imagine driving at 40 mph in third and you want to select second. Check the revs in third, say they are 3000, if you select second then the revs when you change will be about 4000 for the speed you are travelling when you change gear.
What I'm getting at is the need to adjust engine speed to the next ratio as you change. If the synchromesh is really bad or as in some cars, does not exist then double de-clutching will, with practise, give silent changes. This procedure means pausing in neutral as you change , let out the clutch, adjust engine speed then push in the clutch and select the new ratio.
Sorry this post has got a bit long, but I hope it is useful.
Alec