What are principles Christians use for philosophical issues?

Insofar as "principles" here seems to mean a set of tools one might use in philosophical issues, there would be no distinctly "Christian" set of Philosophical principles, just like there is no distinctly "Christian" set of Mathematical principles (nor could one talk of a "jewish", or "muslim", or "Atheistic" set). Within any area of thought the principles of reasoning are laid out within philosophy itself. Thus, if by "principles" we mean "a set of philosophical tools" which guide our reasoning, then it should be clear that such principles belong to the discipline of philosophy, in general, and that these philosophical principles will be the same in any subject whether we are reasoning about Christian theology, or quantum mechanics, or American Law.

On the other hand If what the question means to ask is something more like "What are the Philosophical principles within Christianity?" where the word "principles", instead of meaning a "set of tools", means something more like "notions", "subjects", ideas", or "doctrines", one could speak of a number of specifically Christian "principles". Just briefly I will list a few of the historical issues within Christian philosophy.

First there would be the philosophical issue of theism in general: Namely, the existence of God. Philosophical arguments for theism have historically been given in the form of cosmological, teleological, axiological, and/or ontological arguments. Generally these arguments are given together as pieces of a whole foundation of reasoning. Second there is the issue of the "coherence of theism" or the analysis of the concept of God. This deals with not only the characteristics or attributes which define the "nature" of the concept of God, but also with how they are all rationally interconnected. Generally the attributes are as follows: Necessity, aseity, incorporeality, omnipresence, eternity, omniscience, simplicity, immutability, omnipotence, and goodness. These attributes, far from, arbitrarily selected, usually follow necessarily from either the nature of the arguments put forth for God's existence, or from one another.

Moving more specifically to Christian philosphy, there are topics of creation, providence, and miracle. There are philosophical issues involving the Trinitarian God: Logos Christianity, Modalism, Arianism, Anti Social Trinitarianism, Social Trinitarianism, Functional Monotheism, Group mind Monotheism, etc. Also concerning the incarnation and Christology. Furthermore, Christian theology has it's own subcategories of thought and study such as Theology Proper, Pneumatology, Revelation, Creation, Ecclesiology, Soteriology, Eschatology, Prayer and meditation, Missiology, Patriology, Doxology etc. all of which regularly involve philosophical study. The thought and beliefs which characterize Christianity obviously have implications as well for numerous areas within the subject of philosophy which are not of themselves necessarily Christian subjects: the subject of ethics, for instance, or metaphysics and ontology.

Christianity is a unique realm of thought, experience, and praxis and as such it can, and regularly does employ philosophical principles within it's own reflection. However there are no specifically "Christian" principles of philosophy.