Okay..not going into the full report of what causes PIP. The google bar will take you to those discussions.

First way to avoid PIP is research and being prepared. Some gear has very high ratings but will still have some pip.
Many times it simply has to do with how your body reacts and not necessarily the gear. Other times its ALL user error.

So lets talk basics.
Heating gear is by far the easiest and most common way to avoid pip. I always heat all my oils.
Take a double shot glass or coffee cup, get tap water hot, and put your vials into the cup. Let that water run slowly into that cup at least 5 minutes.
Hot shower during the time helps because it gets muscles relaxed and blood flowing. Dry vial and alcohol swab prior to pulling out the amount needed.
Heated oil flows easily into the muscle and since its thinner it disperses rather than sits as a chemical marble in your muscles...causing pain.

Now, pin sizes has been discussed many times as well.
23 x 1.5 for glutes and 25 x 1 for quads, delts, etc. But I'm constantly getting PM about pip thru a 25 x 1.
As I question the person, they describe the error. Whether its a 23 to the glutes or a 25 to the quads, the biggest problem is how hard you push that plunger which results in the above mentioned marble of oil.
A 25 is a tiny pin so it forces you to pin slower. Simply pinning any oil slower typically erases all pain.
But many get that 25 and then push the plunger HARDER. Slow the roll guys/girls!
On a 23 you should be slower than 30 seconds per cc.
25 is no different and since the 25 is used for most pip gear (props) take extra time.
As your pushing harder on the plunger and making that marble you're also shaking and making that pin move.
As that pin moves nicking veins and tapping nerves, here comes your pip oil to really piss them off.
I like hitting the 25 at 1 minute per cc. NOW, since some oils just simply hurt, so there's also a method as BLENDING.
Its easier to do when you're better skilled with pins and rotation but by second/third cycle you're doing prop anyways.

And no its not only prop that has PIP!
Many of the newer high MG steroids have more pip because there's more of that parent hormone in the muscle at one time to cause irritation.SO...blending is nothing more than getting out the pencil and paper and doing basic math.
Say you're doing test/eq and a prop that has pip. We know that just the test eq we typically pin biweekly, because the long esters.
But were here to avoid PIP.
So if our cycle is 600/600, how bout taking our 1cc of PIP prop and adding half cc of EQ.
The added lil oil helps relieve the irritation know as PIP. Some sources offer sterilized oil specifically for this purpose. It adds a lil oil without messing with math or effecting weekly dosages.
HOWEVER, research oil and how it effects your body in high doses!
And all ya need to do is keep your weekly dose at 600mg eq. And if it comes out mathematically that blending causes the EQ to be +/- 50mg a week, its not a huge fluctuation.
Typically the prop is 4 weeks in front of cycle so after you are positively doing a flat 600/wk.
Reading the labels of your gear is also a key to success or less pain.
Some oils regardless of preparation by heating and pin size just will NOT agree with your body.
If by chance eo hurts and grapeseed is pip free for your body, then use common sence.
Trial and error as well as just Man The Fk Up is your keys to success.
You have to keep in mind that in reality you're shoving a steel nail into yourself and forcing in a foreign substance.