Ask your teachers for assistance. When you look through your design pieces you may think you have a pretty good idea of which pieces are the best. However, while you may be partial to some of your design pieces, your ability to figure out what is good design and what is great design is still growing. Your teachers can assist you in determining what is your best work; that is why they are there.
Look for your weakest links. Once you have a design portfolio of between 10-15 of your best design projects assembled, look over these projects and decide if there are any that should be replaced. If there are some weak work pieces that should be replaced, ask your teachers for suggestions on some projects that you could do to replace those lesser projects.
Do an internship. If your portfolio seems somewhat small and the pieces seem weak, one of the best ways to build your portfolio is to do an internship. Some design schools offer internships with local design agencies. At your internship, ask the agency to give you some "real" work that you could put in your portfolio. Also ask them for constructive criticism of your design work.
Do a peer review. While your teachers are great assets in reviewing the strength of your portfolio, your fellow students will also be able to assist you in determining the strength of your portfolio. Ask a couple of students in your class to look over your portfolio and give you their suggestions. Have them look over it alone, without you looking over their shoulder, and write a sentence or two on each piece.
Choose a nice case for your work. Most schools require that you portfolio be in a presentable case. Do not skimp on the quality of your case, but instead be sure to find a case that will last for years to come. Think of this as an investment in your design career.
Review the requirements for your portfolio and make sure you do not miss any important elements. It is easy for students to get excited about completing and turning a project and then forgetting an important part of the assignment. Review the sheet that your instructor gave you for the design portfolio assignment and make sure that you have included everything in your portfolio.
Aim for your best but do not feel overwhelmed. You will want to enjoy your senior year of college, but the pressure of performing at a professional level may seem overwhelming. Do not submit to the pressure. Do your best work, but also take time to enjoy your senior year and realize that most employers will not expect your portfolio to look exactly like someone who has been designing for 10 years.