1 - Buy day-passes online

Most airlines will allow passengers to buy a lounge access day-pass online.

As an example, you can buy a day-pass from Air Canada for access to their own lounge for $25 if you are travelling on a Latitude fare.

United Airlines offers airport lounge access day-pass for $50 here.

2 - Buy day-passes at the airport

Check directly with your airline. If if your airline doesn't own its own named lounge at the airport, they often have deals with private lounges offering them at competitive prices. As an example, Canadian airline Westjet has partnered with private lounge operators in the various regions it travels (Canada, Europe, Caribbean) to. Westjet offers airport lounge access at very competitive prices.

3 hour access to the Plaza Premium lounge costs $40 - 20% (Westjet discount) = $32.

Some vacation package wholesalers also offer (add-on) lounge access to their customers. As an example, Signature vacations (in Toronto, Vancouver, Edmonton & Winnipeg) sells lounge access to all vacation pass holders.

3 - Buy access to an independent airport lounge

Some airlines do not offer any type of (direct or indirect) lounge access. Other times companies buy the cheapest ticket they can find which means you may fly 10 different airlines and therefore not gain priority privilege access on any one particular airline. These are the times you may need to buy access to one of the independent lounges.

If you travel to different airports, you may want to join one of the independent airport lounge access networks like:

As an example, Priority Pass offers access to 1000 airport lounges worldwide. Priority Pass (sold in Canada) offers 3 levels of membership:

Standard($99 a year). Every access will cost $27 for the member or guests.
Standard Plus ($249 a year). Member receives 10 annual visits. Additional visits or guests cost $27 each.
Prestige ($399 a year). Member receives unlimited lounge access and guests can buy access for $27.

Some credit cards have standing agreements with these lounge access wholesalers and allow you to buy access without having to pay an annual membership fee. As an example, Diners Club Canada offers members access to worldwide lounges at affordable prices.

Here is an example of their Canadian airport lounges you can buy access to. Access to a lounge in Toronto is about $US30.

4 - Get a credit card with lounge access

If you travel a lot, it may make sense to use a travel credit card that includes access to airport lounge (either free or a pay per use without requiring an annual membership to a lounge network).

Credit Walk (Canada) has published an interesting article comparing various credit card lounge access programs.

Sleeping in Airports (USA) also has an article about credit cards offering lounge access.

5 - Buy a refundable business class ticket

Some travel forums (e.g. maphappy, boarding area, view from the wing ) recommend that you buy a full price refundable business class ticket for travel the same day as your regular discounted ticket, use the lounge and then refund the ticket.

I have never used this technique and you should make sure the ticket is still refundable if you use the lounge. I know airlines like United have started implementing lounge access software that will help curb this type of abuse but I know this still works on some airlines.

6 - Buy lounge access from other passengers

You can sometimes buy lounge access from other travelers on classified type sites at discounted rates (eBay, Craigslist, etc). Make sure you check any restrictions that may apply.