I would say Neverbored post is VERY relevant and close to what i would had said. I would add and insist greatly on this... RESEARCH !!! Look at reviews, they are your best friends. Do not pick up your first meeting on pictures alone. Get a very well reviewed SP. One that have only or close to only great reviews. It may be harder to book the more popular girls but in the end you will be happy. Only thing ill go different than him is DO NOT PAY IN ADVANCE. Let the cash on sight yes, but do not GIVE IT. It is usually sugested to use an envelope but i never did actually... never had any problem with this. I usually put money under the hotel phone so it goes out and is visible but to take it she would had to lift the phone and it would take time so i could react. Obviously if you follow my next comments this should not even be an issues ...
AVOID AVOID AVOID !!!!
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You already found merb, no need to go elsewhere. Its true there is gems on these websites as well, actually some of the well reviewed providers here advertise there as well, but since its your first time and your not experienced enough, do not start by there. You don't initiate yourself to snowboard on the pro descent right?

Too much fake pics, pimps promising you stuff that won't happen, even risks to your well being.
If you are going to incall, be there on time, if you prefer outcall, pick a decent hotel. No need to go 4-5 stars (unless provider specify they only go to these) , there some cool rooms in 2-3 stars hotel that do the job fine. Just avoid ST-HUBERT street near the bus station, had issues years ago when booking from there. Not the hotel themselves but girls refusing to go there as these hotels are not well seen. Usually SPs will be more than fine with Chablis/Chabrol unless specified they don't go there. Its an hobby place... not super discreet but always effective.
I would sugest booking by text, when i started i didn't had a cell phone, they existed but they where not "widely used" like now, and i remember how shy i was to call. I could sometimes circle in my room for like 45 mins before actually taking my guts and calling. I hated to talk to the booker. Guys would intimidate me and girls would make me feel shy/ashamed. When i got a cell and started texting oh was it a relief lol. Obviously it may be different for you, its just my personal experience. Now i know most of the bookers and i wouldn't be shy to call at all lol but back then text really helped me.. Only use email if you think about pre-booking an indy many days in advance or such.
As for using another cell or whatever, do it if seeing escorts could be a problem in your regular life. If your like me and tell your friends how awesome your MTL trip was... well don't lol.
Also indies or agencies? Does not really matter honestly... there is gems on both side.