A viewer from Quebec starts the ball rolling on a discussion of maximizing the efficiency of your chimney's draw.
Starting with a small hot fire is always an aid. We need to warm up the chimney before it can draw properly. Chimneys built totally inside the heated part of the house always work better than chimneys that have one or more sides exposed to the cold. Insulated metal chimneys that run outdoors are frequently a problem. The same chimney run up through the house, almost always work well.
Special wind directed "draft assist" chimney caps that rotate with the direction of the wind can help a marginal and sometimes even a major problem. These are not turbines, but more like "weather vanes" that keep the chimney opening always "down wind". One caution to note: you must frequently observe the cap to see that it is in fact still turning with the wind. If you heat with quality dry wood, it will turn for years. If you burn wet wood and create a lot of creosote, the bearings will freeze up rather quickly.
I know of two Canadian companies who supply chimney top draft assists or "wind directed rotating caps": Aim Chimney Sweeps in Penetang, Ontario and Industrial Supplies Ltd (LMI CATALOGUES ) in Rougemount, Quebec. It looks like they quit making the stainless steel cap (#1416) but you can still find all sizes of the 2416 Galvanized Steel cap in their Catalog "C". They are not cheap but my Stainless directional wind cap has done wonders for me for years now. If there is no wind it works no differently from no cap at all, but with the slightest wind, it actually helps to create that initial draft and with strong winds, nothing blows DOWN my chimney, fire or no fire.
There are numerous similar caps that can be found on Google by looking up "Wind directional rotating chimney caps".
If you have to open a window to get a fire started, that is a sign that you need to do some work to get more air to the chimney, not a simple task. For the most through and up to date information on fireplaces and chimneys, you should get the book "The Fireplace In The House As A System" written by Gulland Associates in Ottawa.
For more information on chimney spillage and backdrafting check Spillage Susceptible in this database. Also www.woodheat.org is a non-profit wood heating association worth visiting and follow this link for more information on wood appliance chimneys.