It's not easy, but then it should only be a concern if the value of your coin is high.  If Bitcoin had the mining difficulty it had in 2010, but the price it has now, as an example, you can bet there would be some problems.  But as long as your difficulty keeps pace with your value, you shouldn't be a victim.  

If you do stick with PoW, sensible steps include:

• 1 - not championing CPU-only mining because Botnets
• 2 - not picking an algo that already has huge amounts of ASIC power dedicated to it, because that's the opposite extreme of 1 and equally likely to cause issues
• 3 - not manipulating the price of your coin because you'll make it a more tempting target

There's kind of a delicate balancing act between 1 and 2.  Those are the tricky ones.  3 should be a no-brainer.