Manes
After you've finished rehairing your pony get out a coffee mug and fill it with warm water. Pop it in the microwave. A minute thirty should do it.
Got to the nearest open sink. Take your pony and with one hand, flatten her hair down the way you want it and hold it near the end. Tip the pony upside down and very carefully pour about half the water over the roots. Be VERY careful! I still manage to burn myself on occasion.
Put down the cup and run the pony's roots under cold water to set it a little faster. Let dry.
That should do it. If the bottom still isn't as straight as you'd like you can put the pony in the sink and pour the remaining water over her head. WARNING: Do NOT 'boil' the hair more than twice! It will start to shrivel and harden. That's it! ^_^\/
Tails
These are much easier. Hold your pony by the front feet so your hands will be safe from burns and dump the boiling water over the tail. It'll go straight down in seconds. Rinse the tail in cold water to set it. Let it dry and you're done! :)