First, it must have the consent of the parties involved. The UN cannot impose peacekeeping upon two unwilling parties. It has to wait until they are ready for it. Ironically, the consent can be withdrawn. This happened in 1967 when President Nasser of Egypt requested the UN Peacekeeping force to leave Sinai. As a result, the act was considered as one of the principle factors which brought about the Arab-Israeli War, 1967.
Second, a UN Peacekeeping Operation must enjoy the support of the international community. Take the UN Charter, the mandate to establish the force must have the support of the five permanent members of the Security Council as well as at least of the remaining ten non-permanent members.
Third, the troops of the UN Peacekeeping Operations are provided voluntarily by the member states to the General Assembly. In principle, the costs are to be shared by the member states. In practice, however, many of the member states have not harbored their obligation under the Charter to pay their share of the cost of Peacekeeping Operation. The United Nations is beset by severe and controversial financial problems. There are three elements to the extended UN budget. The first is the core budget for headquarters operations and the regular programs of the major UN organs. Then, there are peacekeeping forces. The third element is called the voluntary contributions budget, which funds agencies such as WTO, UNICEF and so on; it also includes voluntary contribution to peacekeeping expenses.
The UN is entirely dependent on the assessment it levies on member-countries. This assessment is fixed by the UNGA based on a complicated formula that reflects the ability to pay. According to the UN Charter, this is a valid binding to all signatories; members are required to meet this assessment and may have their voting privilege in the General Assembly suspended if there are seriously in arrears.
Fourth, a Peacekeeping Operation is under the command of the Secretary General, who is, in turn, accountable to the Security Council. This is an important principle because if Peacekeeping Troops are thought to be serving their counties interest, they will lose the impartiality which must be at the heart of peacekeeping.