vTiger CRM
open source CRM - if you are really serious about managing clients and need it on a multi user level (if you dont have for it now, plan to scale in the future) i think i have an example install on a spare domain if you want to tinker with it. there are many CRMs out there most are paid and have great support. Oracle has a really good one that has been around forever I can't think of it.
i have customized this to track all of my clients, you can change the status from "warm lead" to "closed deal" with a mouse click.
i use it to track financials, client information, SERP rankings, keywords, and any notes about my client "sounds like Nick Cage on the phone." also track PPC budget and anything else relevant. im also thinking of coding in "attitude" 1-5 radio buttons.
it has been really great esp 9 months into it looking back and re-calling all of our warm-leads and hitting them up again.
the only way i found out about it was that it was listed as ready to install software on my dedicated. i through it on the server and saw how customizable it was and never looked back.
my advice for anyone starting out with client work whether or not you have multiple employees is to find a system and stick with it.
the other trick that i do is i have a fileserver in my office that is set up like this: Driveletter\Accoutns\Clientname --I save all emails in there, website backup, relevant images, reporting, etc so that if I need anything I can access it quickly. I used to sort by domain name but then ran into the issue that a lot of my current clients have several domains (100 1 of them hehe) and that got tricky.
i dont think there is any right way to do it. i stay away from anything with the word "cloud" in it which is fancy way to say "remote file storage with a shitty custom interface and complicated data restrictions." get SFTP for that.
as far as the suggestions above few of them are great, particularly the group calendar and project management software...if you need it.
i am now only in this for a short period of time comparatively but have been lucky enough to have scaled up rather fast. good luck!