Make a inoculum from isolated colonies in a enriched liquid media (like BHI broth).

20% final concentration of glycerol (V/V) is considered.

Mix glycerol well with a measured aliquot of well grown culture (try to use late log phase culture) it needs to be stored at below minus 70 degree C freezer.

Mix well by inverting tightly closed tube and then by vortexing.

Repeated freezing and thawing will burst the cells.

You could use 7% DMSO in place of sterile glycerol. The only advantage DMSO seems to have over glycerol for frozen stocks is that it is easier to pipet because it is less viscous. Use a bottle of reagent- or spectrophotometric-grade DMSO that has been kept tightly sealed.

You can revive the the gylcerol stocks every 8 to 9 months, or once a year.

Scrape the frozen surface of the gycerol stock culture with a sterile inoculating needle, and then immediately streak the bacteria that adhere to the needle onto the surface of an enriched agar plate,  containing the appropriate antibiotics if it is your plasmid containing bacteria. Incubate the plate overnight at 37C.

Return the frozen culture to storage at -70C.

Further, freeze drying, using freeze drier is the best method for longest storage.

It is preferable to use screw capped vials for bacterial stocking.

I think 10% skim milk is also best for long term storage of bacteria.

Again skimmed milk may work as a cryopreservative. None of the methods are as effective as freeze drying.

For people planning to work on H. pylori cultures:

Source: EVALUATION OF DIFFERENT CRYOPROTECTIVE AGENTS IN MAINTENANCE OF VIABILITY OF HELICOBACTER PYLORI IN STOCK CULTURE MEDIA.

Four different cryoprotective supplemented stock media were evaluated for maintaining better survival and recovery of H. pylori type strain NCTC 11637 at two different maintenance temperatures of -20°C and -80°C after one month preservation as frozen stocks.

Our study revealed that H. pylori type strain NCTC 11637 could be better preserved at -80°C than -20°C. The best stock media which supported viability or culturability of bacteria were brain heart infusion broth (BHI)+glycerol+human serum and BHI+glycerol+sucrose+human serum, where the latter yielded the higher recovery rate.