If you want non-specific immune complexes then titrate anti-Ig into your sample. Depending on the sample Ig concentration you will need to find the optimum antibody:antigen ratio and complexes will form.
If you are talking about antigen specific complexes then it will depend on whether your antigen is has more than one epitope per molecule. The more epitopes the easier it will be to form complexex, but you will then titrate your antibody to find the optimal concentration.