AMMO of course!
I suggest breaking in the gun before you start adding accessories on it, that way you will have a better feel for whether or not the parts you just put on/replaced makes a difference or not, and if so, by how much.
Glocks have a HUGE aftermarket parts selection probably surpassed only by the venerable 1911, but truth be known they are really good guns right out of the box, especially the G19 like yours. If you got your G19 without night sights that would probably be the first thing on my list, not so much as it needs night sights, but the stock sights are made out of plastic, and they will actually wear down if you use a tight fitting holster. I bought my G17 in 1988 with trijicon sights already fitted, and those sights are STILL glowing after all these years. I don't know how good those fiber optic/tritium blend sights are or how long they last, but I do know the Trijicons last a looong time.
The stock stamped sheetmetal levers are a joke, and I would get an extended Slide release lever at least. The extended take-down lever isn't nessesary, and neither is an extended magazine release button. Unlike the take down lever though the extended magazine release button is a handy swap but is unnesessary. (Well...more unnessesary than all of the other unnessesary accessories available for your Glock.
Other than that, it's practice practice practice. Extra magazines are always a good investment.
The G19 is such a well balanced gun that there is no reason to mess with recoil spring rate, however I am no fan of Glocks plastic recoil rods (my original on on the G17 cracked) I got a stainless replacement and a new Wolffe recoild sping for that specific recoil rod alone. Staying on the metal replaces Plastic kick you may want to change the slides rear cover (that holds in the firing pin & spring) if you're like me an want to clean the firing pin /spring and channel every now and then. The OEM plastic cover gets beat up really easily in no time. And last but certanly not least is a Aluminm trigger assy. from Lightening Strike to create the best feeling Glock trigger possible.
Do NOT waste your money on an "Over-travel" stop for the trigger connector, That is really a waste of money.
Your gun is a great gun, and one of the best 9mm service type handguns I have ever shot with the exception of perhaps an HK P7. Good luck, have fun and shoot safe. Remember, 1000 rounds on a new Glock is like breaking her in!