I found the best course of action is to hide as much of it in plain sight as you can. The more lies you layer on the harder it is to keep track of it all. Lies need maintenance and you have to be sure to remember them clearly or else someone who's paying attention will catch the inconsistencies.
By making as much of your hobby routines match your real life routines as possible, you give yourself much less to keep track of. I don't use a separate phone since I'm on it all the time anyways. I setup contacts to hide any unfortunate calls or text messages (which I never get anyways) as telemarketers or something equivalent. I use the same computer to browse at home and just use private browsing. I'm not worried about getting "caught" by any of this because none of it is abnormal for me. Everything is setup in such a way that if I get asked about it the answer is rational and consistent.
I agree with everything but the phone.
Using the same phone may remove the chance of being caught with a second phone, but what about your privacy with the SP?
I think the least suspicious phone is the phone that is completely unguarded. It is hard to feel totally confident that you wouldn't get in some kind of trouble if the s/o had your phone to herself for an entire day without warning but.. some of these approaches might be helpful
1. Not saving the actual number in your phone, save a number one digit off. A lot of sp's are searchable through their phone number so you can call them "Cal Tires" till the cows come home but if she is suspicious enough to be looking at your call log, you can bet she's looking up numbers on spokeo
2. if you ever get a call or a text from the sp you weren't expecting, point out the contact that's one digit off.. say something about misdialling or mistakenly calling when you were trying to give someone the number, just needs to sound like an explanation doesn't have to be bulletproof because mistakes are mistakes.
2. Delete text and call log entries with that person, regularly, but not your entire call log or entire text history.
3. If you do not see a regular sp or communicate with her often, go out and get a pay as you go sim card and unlock your phone. Just switch sims whenever you need to