Photo Credit: Courtesy of 3DStockPhoto ( image)
This post is meant to share experience with products that make your products stick to the bed. This is not meant to be a promotion of the products! In other answers I've seen answers hinting to Elmer's glue sticks.
I have been printing directly onto the aluminium heatbed from day 2 of the i3 Prusa clone (Anet A8) printer and the glass beds of the Ultimaker 3 Extended and my HyperCube Evolution using a PVA based spray (3DLAC, but there are more products with similar effects, e.g. glue sticks). This sticks so well that PLA and PETG can only be removed after cooling down the bed completely. For ABS you could use DimaFix which can be used for ABS as it increases grip with increasing temperature where 3DLAC looses grip over 80 °C (according to theory). After trying DimaFix on high temperature beds for printing POM filament (very tough to get this to stick as this is bearing material!) I found that prints stick better with 3DLAC on glass.