Many threads about this on APUG, just do a search.
The juice is:
Freezing: some say it works; might leave crystals on the bottom; producers discourage it.
Topping bottles with propane/bhutane gas (sold as spray cans e.g. by Tetenal): very effective. My choice. Flammable gases, pay attention.
"Accordeon" bottles: Not very easy to clean. Not all very gas-impermeable.
Marbles: work fine but your flask is going to become very heavy.
Squeezing soda bottles: works well if the bottles are made of gas-tight plastic material.
Some people uses glass containers, other (most) use plastic containers. Plastic doesn't break easily but does not clean as easily as glass and, importantly, plastic can be gas-permeable so you must watch which plastic you use.
Plastic n.1 (PET, PETE) is the best for the conservation of development.
Plastic n.3 (PVC) is acceptable for conservation of development, but n.1 is better.
Plastic n.2 (HDPE, PE-HD) and n.4 (LDPE, PE-LD) are slightly permeable to gas, their are unfit for the conservation of developments, are acceptable for the other baths.
Mind that the cap of the container is of the same plastic of the flask. You don't want to put developer in a PET container with a HDPE cap. Use a PET cap.