The following is what I have found out until today.
This should become a general adviser for buying eyepieces.
May I ask you to criticize this? Tell me what is missing?
Thank you!

Amalia
Looking for new eyepieces?

I suggest you...
1. Get informed
Check Cloudy Night's reports, reviews and articles
Use CN's "search"-function
Use Google: Perform a search with the name of the eyepiece and "test" or "review".
Bookmark your results.
Be critical: The reviewers tell their own truth. Your eyes, your preferences
and your scope will probably be different. Don't believe everything.
2. Go to star parties, ask other astronomers if you can try out their eyepieces in your scope.
3. Ask the same at a local astronomy club.
4. Ask in an astronomy-shop if you can try 2, 3, 4 eyepieces over the weekend.
Make clear that you want to buy eyepieces. (Don't give them all back and buy the
eyepiece via internet, please...)
Try out the eyepieces together with a friend.
5. Driving to a really dark observation site is the best improvement of your astronomical system.
Expeditions are fun!
6. Worry about equipment less. Observe more. Enjoy the sky!
7. Are you an absolute beginner?
There are several "philosophies" about aquiring first eyepieces. Here some ideas:
#1 Use first the eyepieces that came with your scope. Get used to the hobby.
#2 Get the Celestron 1.25" eyepiece & filter kit for 136$. So you will start to
understand what eyepieces are about.
http://www.celestron...accessories.htm
Scroll down - I wouldn't go for the 2" eyepiece kit - as for this
money you can already get a real premium eyepiece.
#3 Buy quality eyepieces. Quality doesn't mean expensive. Good quality Plössls
or good quality orthos are not expensive. These simply build eyepieces are keepers,
even if you will continue to observe and you later will buy wideangle eyepieces.
Did you know: The only existing transmission test shows 97% transmission
reached only by a ....* - (and by two Zeiss orthos which also reach 97%).
(* Solution at the end of this text... ).
#4 A wide angle eyepiece does not mean "everything is better" - it just shows a wider field.
From my experience I can say that my orthos show me more details than my wideangles.
#5 Know that you will pay 200 - 300% more for a premium eyepiece - to get 10% more quality...
#6 Enjoy what you own. Never think: I would be happier with Pentax or Nagler.
Beauty is in the eyes of the beholder - not in the eyepieces. Enjoy the sky!
#7 Except for the mentioned kit - buy only one eyepiece at a time.
#8 Let the eyepiece be your teacher - each type of eyepiece will show you
the wonders of the sky in a different way. Empty your mind, open your eyes
and follow your new teacher.
#9 While testing use your brain, think about your new eyepiece.
Check different objects. Check difficult objects. Check the stars
at the border of the field of view. Check the darkness of the sky.
And:
Switch off your brain, listen to your heart, feel.
I think the feelings are more important.
#10 Dare to have your own opinion. Your very own opinion. You are unique.
#11 Ask us: Post your question here on CN - open a thread, which includes:
telescope(s) type
telescope(s) focal length
(maybe your future scope)
eyepieces already here or arriving with the scope
former experiences with EPs
need to wear eyeglasses
main celestial targets
planned budget
ideas or dreams about eyepieces
time of realization
#12 Don't forget the "experts of CN" are people like you and I -
normal people willing to help and share experiences.
Don't believe everything they say...

A "thank you" is always appreciated!
Enjoy!
Amalia

* Solution: 97% transmission reached by the Televue Plössl 20mm!
Same transmission as the Zeiss orthos - amazing, isn't it? Its a high
quality simple design eyepiece!
Some useful links:
Jay Reynolds Freeman's Glossary For Telescope Buyers & Users
Best of the CN Beginner's Forum & How-Tos
An Essay on High Magnification
About Phil Harrington's book Stare Ware
CN Eyepiece Forum: Eyepiece Links of Interest