I am currently planning a wedding for my friend. She was so thankful I had mentioned to her the importance of designing a layout of the reception. i.e., where you want the chairs, tables, decorations, etc. She had also told me that her venue was asking for a layout down the road so they could help set up and get tables and chairs where they need to be so she was thankful I had mentioned it to her now. Her wedding is still a little over three months away but since I'm in Oregon and she is in St. George its even more important we are ahead of the game.
I recommend all brides start designing the layout of their reception four months prior to their big day so they can figure out how many decorations, and table cloths amongst many other things you will need. The layout should not only include tables, chairs, which decoration, centerpiece and flowers go where, but a detailed map of the venue. You will want to decide what door you want guests to come through, and which rooms you will be using on the day. You also need to know where all the outlets are so if you have lights and music you can plan out where it's all going to be plugged in and that will determine which tables go where.
If you are going to have a formal dinner then you need to strategically place where the head table is going to be so all the tables can see you. Remember the head table needs to be rectangular- you don't want guests looking at your back all night and guests don't want to! Have your immediate family sitting at the table closest to you. A lot of people forget about this and in the end only a select few tables can see them. You also can decide what guests you want at each table at this point as well. Remember not to over-cram the tables. Don't ever put more than 10 guests at each table.
Designing your layout is critical so that your reception setup will run as smooth as possible, and those helping will know what to do so they can get it done as quickly and efficiently as possible. Make your layout look very nice. This can take a while so take your time and don't rush this or it may reflect in the appearance and layout of your reception.