There are two different substances you can use. Firstly there is ethyl maltol, which is a crystalline white powder that smells and tastes like candy floss only ten times sweeter. This is used in moderation as it will not take very much at all to make your liquid taste sweet. It comes in crystal form or already mixed in a 10% solution in PG. Secondly you can use sucralose which is a zero calorie artificial sweetener. This stuff is 600 times sweeter than table sugar in it's pure form, so even less is needed to taste. It will usually come already mixed with PG and it can also be obtained in powder form for home mixing. In all home mixing situations, usually a 10% mix to PG is all that is needed as the liquid will reach a saturation point when the powder will just not mix any more and you will end up with crystals in the bottom of your mixture. To help with the mixing process, it can help to sit your plastic dropper bottle in hot water for a couple of minutes to dissolve the powder quicker. Heat helps the process.
Some people use Lorann marshmallow flavouring in 1 dram bottles, although it will be quite costly it is effective.