Ok just an addition to the above guides (which are great and are really helped me initially, so thanks guys). A few things I've noticed: dont use taxis to get to destinations, this will invalidate your time for the mission. I found this out on "sexy time" I used a taxi to get to the pier. When I'd finished the mission it said 100% but then when I hit "A" to view "my results" it said 82%.
Another top tip which I dont think anyone's mentioned (apologies if they have) but you dont have to complete all objectives in one run through.
i.e. on "Chinese Takeout" my first run through I took my time and went for headshots and accuracy (100% Headshots, Accuracy: 70%) This took me 13 mins to do but at the end the Headshots & Accuracy boxes where checked. I then went back through with the automatic shotgun with explosive rounds and stormed though in just over 5 min with less than 10% damage and got 100% :o).
I know this might take longer because it can sometimes mean 2-3 play throughs for each mission but it definately helped me.
Hope this helps GOOD LUCK!!!
# EDIT: To help complete the challenges within the set time here is another tip I found really helped:
Go to the mission start point (you can find this by starting the mission check your location on the map and then quit and head to that location). Once your at the start point call Henrique and ask for a fast car, then keep an eye on your mini map and get your cell phone ready to start the mission as Henrique approaches (once you can acctually see the marker on the mini map). When the mission starts Henrique will just be pulling up infront of you with a nice shiny new car :o)
I found this helped loads and saved precious time having to run around looking for a vehicle.