Gravity is a measure of liquid density relative to water. To have a proper reading, you need to stir/shake the everliving crap out of your wort to make the density uniform throughout the entire solution. Oxygen is necessary for good yeast health, so shake away on every batch (only pre fermentation!!), get a turkey baster or wine thief to get a good sample from under the froth, and you'll always have reliable OG readings.
Primary #1 - Midnight Ryeder (Midnight Wheat and Rye)
Primary #2 - Florida Weiss
Primary #3 - Kane-DOH APA (Honey Citra APA)
Secondary #1 - Downtown Flanders Brown (brewed August 2012)
Keg #1 - Raspberry Florida Weiss
Keg #2 - Cinnamon Raisin Cider
Keg #3 - NONE!
Bottled - NONE!