For the future, use escrow. It just looks like you are working for companies that have no money to pay you (and prob. never will). If a company isn't willing to use escrow, run. They either don't have the money or are planning on not paying you anyway. It doesn't have to be for the whole thing, but at least until you have a relationship with them, use it. Start ups in SV (generalising) are run by kids with little head now a days that think they are the kings of the world, and don't care about screwing you over because that is what they learned from Jobs or Zuckerberg or whatever.
Will serious companies actually post the money to an escrow? I dont have any experience with this directly, but cant imagine any company with say >100 employees would respond favorably to this request
Those companies also usually pay part in advance. If you read a bit about other freelancers process (even here in HN there is a lot of advice) most will ask for some kind of advance.
One of the major advantages of working through an agency is that they have processes in place to asses credit risk and handle collections.
This isn't limited to startups though. I've seen a medium sized company stonewall on payments until the freelancer in question was willing to settle for 20%-30% of the agreed upon price simply to recover something for the time they invested in the project. This was a negotiating tactic for them and when it didn't work they would pay whatever interest rate was agreed upon in the contract, which was often nothing.