
Masteries: A lot of Rakan users are relying on pouring heavily into the Resolve mastery tree (Right/Tank tree). I personally prefer investing heavily in the middle tree for the +Mana Regen and Windspeakers' Blessing. This way, if any skrimishes break out early (when Rakan is NOT strong), I can simply blow my spells as I choose. Atop this, Windspeakers gives me a stronger Heal + Shield for protecting my carry.
Runes: Not sure what most people are running on Rakan for his runes. I personally just use my ADC/AD runes. AD/ASPD/AR/MR; with a good chunk of ASPD for harass (SO nice with Xayah) and lowering my Passive's shield's cooldown.
Laning: Play safe early. I do a lot of pacing around / re-positioning with Rakan ESPECIALLY if the enemy support is CC-heavy. I start with E(shield). Q is too short a skillshot for me to begin with (and can be blocked by minions), and Lv.1 W is useless to me without an escape tool. At level 2, I'll grab W(knock-up). On any enemy duo that doesn't have a very strong Lv.2, I'll go in for the knock-up and shield out if my ADC is in range to harass/trade. For the rest of the laning phase: Watch your cooldowns, keep the wards up, engage hard when ADC is healthy + wards are up + your cooldowns aren't up. Should do considerable damage and get away nearly unscathed (If not, the ADC should).
Mid-Late Game: Your get-away is largely dependant on another player(big thing for Rakan) being in Shield range, so if you're ever alone, try to not be alone for long. Assuming your carry performs well or at the least, decent enough to make a difference later, try to be with them often. That said, don't be afraid of creating an optimal opportunity for your team in the mid-late game. Your Ultimate is EXTREMELY powerful and game-changing (can also use your other skills whilst ulting), so use it wisely and preferably when you expect some serious follow-up. The cooldown isn't massive for the impact it has.
Side-notes: For poke-heavy lanes, I personally run Refillable Potion so any forced Recalls don't leave me blowing the little bit of Gold I have.