First of all, I think it is important to have a long-term resident communication plan. If a resident moves in, and then doesn't hear from you for 11 months, then you have lost that initial relationship that you built with them when they moved in. Some communities call each one of their residents every month to check on them. I don't know if you have to go to those lengths, but at the very least, I think setting up a schedule to call and check in on them a few months prior to renewal is a good idea to start that rapport building.
But before that, we have to actually establish when the renewal discussion should happen relative to the renewal date. I've had situations where the management company sent me a letter just a few days before my notice to vacate was due. That meant I had to digest the new rental rate and make a decision within two days on whether I wanted to renew. On the one hand, that might push the person to renew without really getting into the buying process and shopping other communities, but it also has the potential to really upset the renter, who might feel that management is being unfair by giving them no time to make a decision.
Regardless, if you had a notice to vacate (NTV) of 60 days, then maybe you want to have the renewal conversation 75 days prior to the end of the lease. And then, like I mentioned before, you might want to have calls to the resident to "check in" 105 and 135 days prior to the end of the lease to establish rapport, find out if they have any lingering issues with their apartment, and find out if they are using the amenities, which will help in the discussion to renew. That way, when you get to the renewal discussion (75 days prior), they don't have any problems you haven't already addressed.
And as for communication medium, I highly recommend a phone call first, and then followed by a letter. My theory is that if we can't take the time to call someone on probably their largest cash outlay of the year, then we don't deserve their business.