Depends on the guests, NK. The last con I went to was a two day (Sat and Sun) event, with just fan stuff happening on Friday night. But Stan Lee was a guest at that con (I didn't meet him) on Saturday only so the place was packed, and so were pretty much all of the autograph lines. Sunday was a lot less crowded since most people left when Stan did - which is why I got to spend a good 20 mins talking to Dean about Warcraft that day.
I second the notion to plan out your days in advance. Get to Q&A sessions early to get a good spot. Recording devices and cameras are typically welcomed at them too!
To avoid long lines, try to get on line BEFORE the celebs arrive - in my case, I got on line about an hour before Dean arrived and I was about the 4th person to meet him and get my autograph and photo.
Definitely plan out what you want to say and have signed in advance. Me, I went for one of the photos at Dean's table, as I had nothing super cool to have him sign. (But I have brought stuff in the past, like a toy chainsaw for Bruce Campbell to sign - it's an Evil Dead thing.)
Plan out any special pose you may want if you do a professional photo op. The photos come out great, but it's definitely a "blink and it's over" experience. I didn't plan anything and I kind of regret that, but I did get to rib Dean about his choice to play for the Alliance on Warcraft then.
Bring snacks or plan to eat at the venue. Going outside of the venue is risky - could take forever to get back, hard to get a cab, impossible to get a parking space, etc.
If you are staying at the hotel, at least here in the US, most local takeout places won't mind delivering to the hotel. That's what we did for dinner at my last convention anyway. SO much easier and cheaper than eating out/at the hotel restaurant.
Bring a variety of payments - cash, credit, etc. Most vendors can do credit nowadays, but you never know.
I always like to circle the dealer room once or twice before buying anything. You may just find things at other tables for cheaper!
Don't be afraid to ask Dean for a hug (respectfully, of course). He's an awesome hugger!
Extra batteries and memory cards for your camera are a must! I must have taken several hundred pics at my last one. Especially during Q&A time, they really pile up fast!