Joe Carter has some interesting suggestions here on how to build a theological library on a budget. Here are a few additional and somewhat random thoughts on the topic.
Building needs to be intentional. A good library is rarely, if ever, going to just happen.
If you are in seminary, try to avoid the temptation to simply borrow textbooks rather than purchasing them. Professors who are generally familiar with what is out there, usually choose books that they believe are worthwhile. I know most seminarians are on a tight budget but consider textbooks as an investment.
Do not confuse building a library with actually getting to know the books in your library. Get what you need and use what you get.
Create and maintain a wish-list (e.g., Amazon wish list). Have a list ready to go when birthdays, Christmas, or just someone one who wants to do something special comes along. Populate that list with a mix of pricey and less pricey options so that even those on a more modest budget can participate.
Review books. Many publishers and some authors will give you a copy if you would be willing to review their works. It helps if you have a platform like a podcast, blog, etc. but these are not essential.