On these bikes it is pretty easy to get to the clutch and it would probably benefit from being replaced anyway so I would take the cover off and fix the clutch manually, personally. All the same, for unsticking clutches quickly I like revving it up in neutral, holding the back brake hard, and then dropping into gear. Works every time, no fuss. Can't go wrong with the other suggestions here either.

Wonder if I might piggy back here...tks   I ave a 69 DT1b 250cc and when I kicked it over at first...more often than not it went into slipping and almost taking my shin out.  Sometimes it would catch and roll over bike nicely and then sometimes it would damn near amputate you leg.  Kind of like Lucy taking away the football before charlie Brown could kick it.  I put her up for a long time....but finally broke into the right side.  The clutes and friction plates were all stuck together and nasty.  And when comparing the parts to what I should have...I think I'm missing a flat bearing hear or there and may a friction plate or someting....something is messed up...now I have reassembled (after using diesel soak to get plates apart...) but now it won't even roll over the motor in any gear for any reason and still doesn't kick right...I'm interested in where I can buy a clutch or clutch kit for it where everything you need comes and make it like brand new again.   Tks    I think the plates with the fiber on them is still there but so mushy or not normal.  Been sitting. Tks

With the DT1 & other smaller models it should turn the motor over even if you had no clutch plates as it kick drives via the back of the clutch which turns the clutch basket & hence to the crankshaft. If it's slipping through it sounds like something not right in the ratchet engagement of the kick gears so can only think to look there. Link below if the pics don't come up good enough. The flat bearing is a two piece iten with a fairly thin washer on one side but has to be fitte on the right side so the bearing runs between the big flat washer & its own thin washer other wise if fitted wrong the rollers will be running on the backof the gear--not shown in the clutch pic--gear is shown in the kickstart pic. Tis confusing i know.With the DT1 & other smaller models it should turn the motor over even if you had no clutch plates as it kick drives via the back of the clutch which turns the clutch basket & hence to the crankshaft. If it's slipping through it sounds like something not right in the ratchet engagement of the kick gears so can only think to look there. Link below if the pics don't come up good enough. The flat bearing is a two piece iten with a fairly thin washer on one side but has to be fitte on the right side so the bearing runs between the big flat washer & its own thin washer other wise if fitted wrong the rollers will be running on the backof the gear--not shown in the clutch pic--gear is shown in the kickstart pic. Tis confusing i know.
www.partzilla.com/catalog/yamaha/motorcycle/1969/dt1b
www.partzilla.com/catalog/yamaha/motorcycle/1969/dt1b

My 2 cents, If the bike seems to be in a generally unmolested condition and just sat, then I would suggest draining the oil and refilling with Mystery Oil and letting in soak for a few days. That stuff helps with stuck engines and mechanisms. Then you might want to get it into 2nd or 3rd and roll down a incline and pop the clutch. Whatever you feel comfortable with to help separate the plates. If you get it lose. Drain and fill with engine oil.
Always good to look at the clutch so you know what's there. It is deeply comforting feeling when you get your machine ready to ride and you know that everything is good from fender to fender because you looked at, fixed, or replaced everything. 
Good luck!