As a venture capitalist, you should not use any lockups or other exclusivity agreements with potential investments. These are one-sided arrangements that only favor the venture capitalist and can be harmful to the potential portfolio company. Top accelerators, such as YC, have warned portfolio companies against entering into these arrangements as a result, and this can be harmful to your reputation as a VC is you using such agreements.

The best alternate is to build a strong relationship with the portfolio CEO, encouraging them to save you position in the round based on the strength of your relationship and the assistance that you have provided.