With all the posts on which (seat,exhaust,shocks,etc.) to buy, I decided to start a post to cover it all. This should be for someone buying their first every Harley, or switching families within Harley. For example going from a sportster to a touring bike, or a dyna to a sportster.
First thing other than cosmetic items, keep the bike stock for at least 2,000 miles. Trust me, that seat,handlebars,shocks that came with the bike might actually be ok for you. Ignore this advice if you have 44" apehangers that you cannot reach or a solo seat and you ride two up. Obvious exemptions apply here. With a brand new bike it takes time for things to settle, ride it and see before you waste a bunch of money. If you have purchased a pre-loved bike let your body get used to it, same thing applies to new here. I think that by just riding the bike for a couple thousand miles, your body adjusts to it. This is how you find out that the seat is too soft\hard or the handlebar wrist angle is just wrong for you.
Now armed with all of this data you have discovered about yourself and your bike you can research. Instead of asking what bars do you like, you can ask what bars keep your hands at a flat angle. Or which seat out there moves me up closer to the handle bars.
Also the classifieds here are a great resource to buy used. I picked up a set of slip-ons for my 2015 roadking that looked like they just came from the manufacture for less than half price of the best online deal I could find. This can be a great way to try things out and not get burned so bad. If you buy used and absolutely hate it, chances are you can re-sell it here and lose little if any money. Try doing that with new parts!
Ok, time for others to post up there pro tips.

 

What's that phrase..."shut-up & ride" or something...?

 

Are we going to ride, or are we going to dick around all day?

 

That's the one...or "let's surf the internet and peruse online forums all day."

 

Maybe you were thinking along these lines:

 

My advice is to begin moding before you even drive off the lot with a new bike. On your way home from the dealer, start thinking about Reinhart or Bassani or Screamin Eagle.

We only live once. Mod away and have fun! If you mod the bike into something you don't like, then it's time to trade in and let the fun start all over again!!! Wooohoooo!!!

 

Or shop for mods before you buy the bike lol. I know mods i would do to each class of harley lol. Changes weekly though

 

Or buy a new bike and start tearing it apart the minute you get home and spend at least 2 years modding it before you ever even ride it. Sure seems to be the popular thing to do on these forums!