Basically, you show proof that the relationship is genuine - that you two really love one another. If the visa officer believes that, then he or she will believe that the applicant is not marrying just to move to Canada.
If the application is being processed at a 'difficult' visa office, or there are a lot of red flags, then it might be a good idea to mention somewhere that the two of you have discussed what to do if the application is rejected, and have agreed that you will then go live in the applicant's country together.