Hi, oh!
In my case, for any relevant character i have a document where i wrote its appearance, its personallity and a brief summary of its past. I.E:
Name: Jafren
Age: 34
Physical desc: He looks older than he is. Black hair with countless silver lines. He's not tall nor short. Usually wears comfortable clothes and a fancy hat.
Personality: Quite distracted, but very professional at his job. He's a bit too serious, but he always have a time for a good joke. He has a tick which is that when he's in a good mood, he usually touches his nose.
Past: He was born in Liaron. His parents, from a decadent noble familly, raised him as a noble, thinking they would gain the status they lost. Unfortunatelly, they both die in a plague and Jafren, with only 12, had to find a way to survive. Luckyly he found a man, Diorth, who teached him all he now, making him the best information dealer of Befassen.
That would do for a secondary character that i can use whenever i need to. As i spend some time (~10min) in creating him, now he's not only a name. He's an entity with personalyty and a story behind him, so it's easier, for me at least, to remmember him. I may forget some details, but that's why i wrote them 
I hope that will help, not only you, but everyone who reads this 
Tell me what do you think 
PD: Thats a fictional character i just created for this example.