How many times have I struggled to clear blocked drains with long sticks, an old cracked rubber plunger, or waiting until I could get to a shop to get a commercial drain cleaner? Those days are long gone and it was all a lot easier than I could ever imagine.
I have long applauded the amazing abilities of vinegar and bicarbonate of soda (baking soda) separately. And the two combined make a perfect drain cleaner. Even for standing water. You can also use them to "freshen" drains to prevent problems. Simply place 1/4-1 cup of bicarb and the same again of white vinegar in your sink or drain. If you can, place the plug in (not for standing water). Leave until the drain is clear. You can rinse with a kettle of boiled water if you like.
Note: Many drains are blocked by an accumulation of grease and then debris which settle in it. For this reason you might be able to get away with a kettle of boiling water which will melt the grease away. But the vinegar combo is more effective in the long run.
I have even used this in an outside drain where I was told I would literally need to scoop out the gunk blocking it and then use a commerical product. Since I couldn't get to the shops for another day I figured I would give my method a go. Two cups of each and we were away! And we haven't had a problem since.