Question about Shifters I have a bike with lever/downtube shifters. I hate them, feel like they don't stay in gear for more than 10 minutes at a time, and would prefer a bike that has preset gears. Is there a way to get the shifting mechanism on my bike replaced to something like a thumb shifter? 

I'm not looking for opinions about the best kind of gears.
There is a nice instruction video from RJ The Bike Guy on youtube on how to convert from downtube shifters to brifters. Perhaps that can be some idea for you.



In case the video is taken down, I'm posting a written answer.
Yes, it is possible to convert from downtube shifters to brifters. Here's what you need:


brifters compatible with the cassette and front chainrings on crank you already have, or
brifters, cassette, front chainrings with crank, probably derailleurs in case your current are not compatible with new cassette and front chainrings
new brake cables with housing, new shifter cable housing (shifter cables are delivered with the brifters usually)
"downtube cable stops" (can be found in the internet) - the cable stops that allow the shifter cable to go pass the current shifter bosses to the steering bar and the brifters.


The procedure is pretty straightforward:


you remove the components you need to remove (at least the bar wrap, current brake handles, cables, downtube shifters), 
install the new components,
install the cables, 
adjust and finetune everything,
and enjoy your upgraded bike.