When do I use 'de' to express possession?

Usually you would use the particle 'de' 的 to express possession, as in 'my clothes', 我的衣服- wo 'de' yi fu- or 'the clothes I wear', 我穿的衣服. However, when talking about close personal relationships or institutional relationships- for example family/partners or school/work, respectively- you leave the 的 out, as it sounds distant. So 'this is my girlfriend' would be 这 是 我 女朋友- zhe shi wo nu peng you- rather than wo 'de' nu peng you.