I rest a wooden spoon, handle side down, into the neck of the flask, that helps deter boil overs.
I also use a spray bottle filled with water, which is far and away the best boil over tool at your disposal.
Kegged and serving -Rye IPA, Kohatu Pale Ale, Brown Forge, Marilla Wheat
Bottled: Too many to list
Fermenting: Flanders Red, Solera Lambic, Sour Rye Solera, Consecration, Apfelwein, Apple Cider, Nelson Saison, Sage Saison, Farmhouse Brett Saison, Little Bo Pils, It's a Celebration Bitches