What are some ways to help motivate ARNG soldiers who failed their PT test to pass their next one?

Send one packing - the one with multiple flaws and failures, make the General Discharge their failure to pass their PT test as the final reason why they're gone. All the others that can be motivated will begin to toe the line. (For the Montgomery GI Bill program or Post-9/11 GI Bill program, you need an honorable discharge.)
Those that can't be inspired will leave or grudgingly improve and your graders had better not be a part of their 'good old boy' network.
Of course this method only works if the unit's leadership aren't a total bunch of loser, fat, pudgy chairborne rangers.

Pass this on!!!

If you or another (willing) Soldier live close enough, encourage a "workout accountability buddy" scenario. Almost all humans are more likely to do a task (go to the gym, show up for work, shower, you name it) if they know someone will be there checking on them. By having a workout buddy, it makes it 10x harder to make excuses not to go today. Now you dont just have to tell yourself you are too tired/busy/whatever, you have to tell someone else. It makes a TON of difference. Especially in a scenario like AR/NG where 1SG isn't holding formation everyday and asking pointed questions about who isn't doing PT and why not. (Which is essentially 1SG acting as EVERYONE'S workout accountability buddy.)