The Pre-Flop Checklist - Evan Jarvis (Gripsed Poker Training)
Critical Fundamentals of Pre-Flop Play - CrushlivePoker
Mastering the Fundamentals: Preflop Strategy - Jonathan Little
Important stats about you and your opponents, that are more easily tracked playing online using HUD software.
VPIP is a stat that tracks the percentage of hands a player voluntarily puts money into the pot pre-flop. Includes calling the big blind, calling a raise and raising.
Does not include paying the blinds and antes which are involuntary.
PFR is a stat that tracks the percentage of hands a player raises and re-raises pre-flop.
UPFR is a stat that tracks the percentage of hands a player raises pre-flop when action is folded to them.
See basic strategy for more on selection / hand selection.
See basic strategy for more on hand ranges.
Ask questions pre-flop - isolate.