I would definitely run this thing off of your windshield, not mounted upon the dash. If you are attaching it to your windshield, it should have plenty enough suction to keep it in place. Ensure you read the direction on how to make it stick, but two pieces of advice on that note:
1) Ensure both the mating and suction cup surfaces are perfectly clean of dirt/lint/debris. Use some Windex or other cleaner to make it so.
2) Ensure the suction cup is moist (you can use spit, but water should work just fine) when you go to apply it to your windshield just prior to affixing it. This will ensure it has a tight bond with the windshield.
As for a list of states which have a windshield obstruction law, I found these states have one:
- District of Columbia
- New Jersey
- New Mexico
- North Dakota
- Rhode Island
- South Carolina
- South Dakota
- West Virginia
While it may be illegal to mount devices on the windshield in these states, enforcement of these laws is very slack. I live in Virginia and have never been stopped for running a mounting on my windshield here, though when I have my State safety inspection, they say it cannot be mounted. If anything, it's a secondary offense type thing, which means they could give you a ticket for it if they have pulled you over for a primary offense. It's been my experience that a police officer would probably just suggest you remove it from your windshield and not worry about the paperwork.
It is also of note that these states have a law which dictates where you can actually have a windshield mounted device located (each state will vary as to where):
If you are still having issues with your suction mount or do not feel like tempting the Fates to mount it to your windshield, Velcro on the dash works very well to mount things. Back in my early days, I mounted a cassette deck to the bottom of my dash using it (yah, really dating myself), and that thing was no where near light. Hook and loop fasteners are great for such a thing, but personally I'd just run it off of your windshield and call it a day.