YCombinator startup school is a great forum to network with founders and apply to jobs. I would also be a regular on Angellist. I would say though—the best way would be to build a personal relationship with the founders.
Whilst we do not offer jobs, you can gain Experience and Skills by helping a tech startup at www.skilledup.life. Then use these Experiences and Skills to land a job from elsewhere.
All the opportunities on SkilledUp Life are from tech startups. Currently those from the UK, USA, India and Sri Lanka.