Brain heart infusion broth with 2% glycerol. Then store in -20 °C. This will maintain your culture for 4-6 months.

we preserve bacteria in broth (brain heart,trypton soy ) with 20% glycerol in - 80 c

I will prefer to go with large size bacterial inoculum for preservation. The chances of bacterial death during and after preservation are evident so we need to take few more steps to get viability after preservation. First as mentioned, try to take large size of inoculum, so that some bacteria will revive even after death during preservation. Second, do not give direct shock to bacteria, by keeping it in -80 degree C. instead give them step wise shock, by first keeping in -4 for few hours and gradually decreasing the temperature. This can also reduce killing of bacteria during preservation. The rest is same for shorter duration, you can go for preservation in Glycerol, while for longer duration, you should try lyophilisation. I hope this will help.   

Trypticase broth with 15% glycerol works very well. Inoculate heavily and preserve it in -70C immediately . You can preserve it for almost an year.  

Survival of Streptococcus pneumoniae, Haemophilus influenzae,

and Moraxella catarrhalis Frozen in Skim MilkTryptone-Glucose-Glycerol

Medium. By Tarja Kaijalainen,Esa Ruokokoski, Pentti Ukkonen, and Elja Herva.

Please go through an aforementioned article. Iam sure you will get answer to your query.

first make a massive culture onto petri dish with Mueller Hinton Agar. Then, you harvest it with 15 ml of sterilized Skin Milk 20% w/v, and dispense the bacterial suspension into cryovials and preserv to -70°C. 

You can maintain your isolate by inoculating heavy inoculum in brain heart infusion broth with 15% glycerol. Then preserve in -80 °C

inoculate a heavy inoculum ito TSB  and 20% glycerol and preserve it at - 80 c

Thanks so much for all , but I don't have -80 C can I maintain my isolates with 50% glycerol at -20C.

You can use either TSB +15-20% glycerol or BHI +15-20% glycerol or skimmed milk as mentioned above. If you store in -20C, it'd better do subculture the bacterium every 2 months.