Most liquid feeding is done weekly or bi-weekly and powdered or granule fertilizers are done monthly or every couple of months. Neither is better, it's just a matter of personal preference. I would just follow the directions on the product, since each product is a little different. But your are correct, the soil should have all the basics and your just topping off. Nitrogen is usually the first to go which is why people apply worm or fish teas, since those are good natural sources of nitrogen. Later on the plant starts to use other things like Phosphorus and Potassium.
To be honest, it's a bit of a guessing game and most gardeners use their best judgement. Commercial farmers on the other hand will do regular soil and tissue testing to find out exactly what their plants needs. You can buy $5 soil kits and do your own tests - it's not as good as a lab analysis but still useful none the less. Just do a simple soil test three or four times during your first grow and you'll see what gets used up and when.
As for what are the best brands... well each seem to have their die hard fanatics but if you read the ingredients you'll notice they all contain pretty much the same thing: Nitrogen, Phosphorus, Potassium, and some basics micronutrients (Mg, Ca, Iron, etc).
If you really want to get into the science of it and learn which ingredients are really better, google the phrase (without quotes) "fertilizers + site:.edu" which will pull up articles published by universities. In other words, no forum bull, just real studies from people who really know what their talking about.
As for flushing... an organic garden doesn't need to be flushed unless you happen to accidently over apply your fertilizer. If that happens then you flood the soil with fresh water so your plant don't get nute burn and die.
Hope that helps,
- Vin
