Complex sentence: Its sales, which had for some time been substantially lower than those of the NME, entered a serious decline, and in 1999 it was relaunched as a glossy magazine, a move which hastened its demise.
Split into simpler sentences: Its sales, which had for some time been substantially lower than those of the NME, entered a serious decline. In 1999, "MM" was relaunched as a glossy magazine, a move which in retrospect hastened its demise.