Single Nail Flatpick
What about fast passages ? There are classic techniques "apoyando" and "tirando" used by classic guitar players. I use them as well, but on acoustic guitar with metal strings, I use also a technique of playing with fingernail of my first finger. If you have week fingernails, just paint them with transparent paint for fingernails (You can use colored too, but ...). Anyway, after few months you'll be surprised how they become stronger. Really, I've done it by myself, it works. Now, about technique. Put your thumb on your first finger in such a way that now the first finger is used as a flatpick. That's it. Try any scale just to warm up.

Double Nail Flatpick
Here is the trick I like to use. Playing with first and second finger simultaneously on two strings using them as a flatpicks. This is a little unusual technique, but it's very effective. I use this technique for playing melody on two non adjencet strings (I hope you're understand this), for instance, the first finger is playing on 5-th string and the second is playing 3-rd string, and so on... To get a feeling, you can put the point of your thumb between first and second finger and practice a little in this way. When you get adjust to distance between fingers, you can remove your thumb (not permanently), and place it somewhere else. Here is a part from my piece Ubavo Oro . Note : the piece is written in 7/8 measure and tuning is Open G major .